Or perhaps we are stricter when raising girls, so they become more compliant.

"don't be so loud, that's not lady-like" "don't play in the dirt, you'll ruin your dress" "it's uncomfortable, I know, but you have to look pretty" "girls don't like sports, here, sit down and play with your dolls"

It's subtle things, but a million of them may make a difference. There's a lot more rules for girls than boys, they are used to spend their time in quieter activities, etc.