On your screenshot the first subreddit in the drop down list says it has 177k subscribers according to the reddit ads site but if you go check that subreddit (https://www.reddit.com/r/The_Division/), it has only got 24k subscribers. So it's the same scenario as the_donald
The 3rd subreddit on the drop down list on your screenshot says it has 26k subscribers but when you go check it, it only has 3k. So it's the same scenario as the_donald
The 4th says 23k on the reddit ads site but it only has 0.5k. So it's the same scenario as the_donald
The 6th says 60k on the reddit ads site but if you go check the actual subreddit itself it only has 2k subscribers. So it's the same scenario as the_donald
If you go check all other subreddits you will find that they all say X subscribers on the reddit ads site but only have x subscribers on the sub. That will be true for every single sub. So they are all the same scenario as the_donald
Given that all subs show a much higher "subscribers" count on the reddit ads site than the number of subscibers on the actual subreddit, maybe the "subscribers" on the reddit ads site doesn't mean the same thing as the word "subscribers" listed on a subreddits top right box
And that kind of makes sense. The subscriber count you see at the top right on a subreddit isn't that important to an advertiser. I'm subscribed to many subs that I never ever visit again - so if an advertisers pays on the basis of my subscriber count contribution they're wasting their money as I'll never see their ads.
What matters is how many people actually visit the sub on a regular basis. That's a real subscriber. I visit the_donald every couple of days so I'm a subscriber as far as it matters to an advertiser, but I'm not actually subscribed to the_donald so I can't show up in top right subscriber count.
So it seems fairly obvious that "subscriber" as it is defined at the reddit ads site is something that is meaningful and useful for advertisers. It probably tells them how many people actually visit the_donald on some kind of regular basis. It's meaning to an advertiser is different, and much more useful, that the subscriber number listed at the top right of a sub, which is there for the users rather than advertisers