Chivalry is more than bravery- it's more of a "code of conduct."

I think this implies that they have a moral compass that they go by, and defend what they believe in. Not to mention in more recent times, chivalry is associated with being a gentleman and having manners, and treating others with respect.

I also kind of disagree about it being a behavior- chances are, if you're not a brave person, you're not going to suddenly act out and do something brave. Look at Pettigrew- he hid behind his friends, then hid behind Voldemort, and never stood up for himself. Perhaps he admired bravery, and wished he could be brave, and that's why he was sorted Gryffindor.

I think bravery helps determine your behavior if anything- this is why Hermione can show up to the wizarding world at 11 years old and act like she owns the place with what she knows, when I think a lot of muggleborns would be both fascinated and absolutely terrified to up and leave what they know to go this school where they know nothing, when a majority of attendees have been in the magical world all their lives. This is why Ron would sacrifice himself on the Wizard's chess set in SS, instead of just trying to figure out another move that could possibly cost them the game, but keep them all safe. This is why Harry would put himself at risk time and time again, despite knowing he could've gotten help from someone with more experience, be generally sassy with Snape, even though he's an intimidating teacher with an obvious distate for him, AND still find the time to annoy the Dursleys, even though they treated him like garbage since he was a baby.

In Snape's case, he knew his risks, and tried to back out at one point when it was becoming too much. He wasn't boldly showing his bravery, and he had a personal motive as to why he did what he did- yes, it was ultimately a noble act, but it was based in selfish reasons.

As an afterthought, Gryffindors are also supposed to be strong-willed and often times short-tempered. Stubborn, brave, hot-headed, chivalrous. Not a bad round-up.

At that rate, of course students are going to encompass qualities of other houses. Even though I'm sorted Slytherin, I'm stubborn, very intelligent, and hardworking. I have traits of all the houses, as most students will, but it's how you use your skills and what you desire to make out of your skills that ultimately determine where you're sorted. This is how you see brave students from other houses, sneaky/cunning Gryffindors, a seemingly air-headed Ravenclaw, a pompous Hufflepuff, and a brave Slytherin.