My experience is from 25 years ago and not in this country but I went to math / science school from 5th-8th grade and there were 6 of us girls and there were 9 of us girls and 21 boys. Huge difference too and we all got into the program based purely on results of math entrance exam. They took in the top 30 kids and that was it. From all our friends kids, I can see how boys are a lot more mathy than the girls, though I'm generalizing here. Math might not be as interesting for a lot of girls so they do not put as much effort into it, perhaps?