I'm more in your camp of "it's culture" but I think it's a lot to do with the amount of attention a parent can pay to their kids education. Some of that is cultural but sometimes it's just a time constraint. Either due to financial constraints (and time spent working), single parents or whatever.

Kids obviously vary in aptitude but the ones that actually get the chance to excel are usually the ones that are getting an extra push at home. Some kids you just can't stop though and excel regardless.