I volunteer each week in my son's second grade class (he tested last year into the 5th grade on the WJ test; he's fairly bored with the math).
I noticed overwhelmingly that the girls in the class freeze and get this "deer in the headlights" look when we do math. Even very simple math. The boys who are weak in reading still seem more confident and even better with math than the brightest girls. Several boys are very good at math, and I haven't observed any girls who are really good.
I guess it's cultural/gender-based. I got a 5 on my AP calculus exam back in the day- we had only 3 girls out of 20 in my h.s. AP calculus class, and I was the only girl to get a 5. Several of the boys did.