I'm gonna quote a well written post by /u/Mrs_Frisby, who answered how the gender pay gap is expressed very thoroughly in a different thread.

"There are a couple different ways. The two biggest are gaps in jobs with subjective performance metrics and pink collar jobs. So I'll give each one a post.

1: Subjective performance metrics.

Because laws have been passed requiring equal pay for equal work, you can't just pay women less for X units of identical work. Which was something you could do in my mother's time and was generally "justified" with the notion that men work to support families but women only work for "pin money" so women don't "need" money as much ergo shouldn't be paid as much. But basically if you have an assembly line and you pay a man $Y per widget and a woman $X per widget you are going to lose a lawsuit so that doesn't happen anymore. Well, not much.

But what if it isn't an assembly line? What if its ... a lawsuit? You've got a team of lawyers working on it. You win. How much credit for that win goes to each member of the team? And by extension what should their bonuses and raises be? Whose performance should get them on the short list to make partner? Thats a much trickier question than "how many widgets did you make per hour?". Even if you have absolutely no biases playing into it of any kind whatsoever proportioning the credit completely fairly is a nearly impossible task. You would have to have watched every move of every team member for every moment of the case AND have godlike omnipotence which would let you determine with accuracy how effective each decision each team member made was. Only then can you accurately quantify everyone's contribution and accurately convert it to dollars.

Not only do bosses not have this information, they do have a crap load of personal baggage and biases. Even in a team of all straight christian white guys maybe one guy is more charismatic than the others and is able to create in the boss the perception that he is carrying more load than he actually is so the boss inaccurately attributes a larger portion of the credit to him than to the quieter guy who did the actual heavy lifting. Then raises and bonuses come out and showboat gets the lions share while the worker bee guy gets a pittance and goes to the bar for some angry drinking. Its not fair, but whatchagonnadoaboutit?

Now throw gender in the mix. In order to create visibility, to get the bosses to see your contribution, the first step is to be heard. It doesn't matter how awesome you are if the bosses don't realize what you are doing. Gender roles dictate that in mixed company women let the men speak. Men will talk over women in a manner that is socially unacceptable if done to men. Women who insist on being heard face social consequences. They will experience backlash, non-cooperation, harassment, etc in retaliation for being "bitchy" and not adhering to feminine gender roles. Here is a pbs article summarizing hundreds of studies of speech patterns in mixed company. To quote one of the researchers:

The talkativeness of women has been gauged in comparison not with men but with silence. Women have not been judged on the grounds of whether they talk more than men, but of whether they talk more than silent women.

Numerous studies have also shown that if a group is 50/50 men and women, and women talk as little as a third of the time, the men in the group will report that the women "dominated" the conversation. And will be miffed about it. The really fun one is the studies done on teachers who think the other studies sound reasonable but flatly deny that it is happening in their classrooms because they aren't sexist. Then researchers record all speech in the classes for a few weeks and show the teachers cold hard statistics of boys vs girls talking and they realize that it isn't even close to 50/50 and that their own perception is quantifiably wrong. One such study added an interesting element of tracking the teachers's gaze and showed that they were almost always looking at boys in the class when asking questions, encouraging the boys to respond. From my own experiences, managers do the same thing. How the fuck am I going to get visibility for my contributions if I'm not allowed to talk in meetings? How the fuck can I contribute to my optimum potential if people are mad at me/noncoperative with me because I spoke up in meetings? #ItsATrap

Now, there is one other option to raising your visibility to management besides speaking up in meetings. You could go talk to them one on one, make a case for yourself, tell them what you did, and ask for a raise. I'm sure you've seen the wage-gap deniers mix a little victim blaming in there. Women don't ask - so what gap exists is our own fault, right? (as if that would make it ok).

Well, funny story. Turns out that studies on raise negotiation reveal that male bosses fucking hate it when female employees try to negotiate a raise. Male employees can ask a male boss for a raise with no social penalty. Your boss might grant it to you, or not, but he won't hold asking against you. But if I, as a woman, do the exact same thing, I can get in hot water. This is even more important for initial salary negotiations because future raises will generally be percent based on where you start and attempting to negotiate for a larger starting salary while female significantly decreases the chance that you will get the job at all. He will perceive you as "pushy" or "bitchy" and not want to work with you. A man with an offer in hand who asks for 5K more is not risking losing the offer in hand. A woman in the exact same situation is risking losing the offer in hand.

So if I can't talk much in meetings, and I can't ask for a raise, and I'm not working in something with easily quantifiable performance metrics that would allow me to sue if I'm not paid fairly ... How would it even be logically possible for this not to cause a wage gap?

But thats not all! Take my field, programming. If you have me and a guy sitting next to each other working on a code project the default assumption of any and all observers will be that he is doing the hard parts and I'm doing little or nothing. The technically adept folks will realize that its pair programming or that I'm tutoring him. But the manager walking by won't. Male and female huddled around one computer? Default assumption is that he is helping me. You've seen this meme a million times, customers go to a helpdesk for an IT problem and dismiss the women working there to seek help from the men working there because this is a real problem ... Sure its harmless when its a customer doing it. But managers do the exact same thing. In the absence of evidence to the contrary they will assume that my contributions are minor.

The combination of people assuming you are not a main contributor by default, and socially not being permitted to strut your stuff, is formidable and leads to a significant wage gaps in jobs where performance metrics are subjective. A frequent way this is minimized or hidden is by tracking wage "by title". In these kinds of jobs roles are often fluid with everyone expect to pitch in at anything as needed and titles are less a definition of precise responsibilities and more ... a random string of letters that gets HR to give you a paycheck with the negotiated number on it. So you can have a coding team - or a law team - where someone with a low paid title is carrying the team.

For any women in tech reading this the best way I've found to deal with it is to switch companies every few years. The poaching company will offer you a raise to leave."