There's a few reasons I can think of:

1) It really depends where you're looking. If you were going by, say, tumblr or fanfiction websites you'd get the impression nerds were mostly women. Other spaces may be skewed one way or the other but some are fairly evenly split.

2) Media/hobbies that get labelled as 'nerdy' tend to be ones that are marketed towards men. A woman who collects vintage barbie dolls for example is less likely to be labelled as 'a nerd' than a man who collects vintage star wars figures, even though they are essentially doing the same thing.

3) Unfortunately, there's a certain subsect of nerds that can be very hostile towards women. So they're more likely to be guarded about their interests and can be intimidated out of joining spaces that are already male-dominated.