I have always had a lot of girls on my math teams. As a social scientist I have to say that it's probably just a coincidence plus the fact that I have daughters. But it could also be that my more reading-oriented approach is a good fit for many girls.

If it's not too much bragging, you can find a story about some of my girls at
http://mathprizeforgirlscommunity.blogspot.com/2012/08/newton-north-math-prize-shout-out.html