The reality is that there are simply very few women that hold the qualifications necessary to be successful in the startup world. When you do find one, the competition for their services is incredibly fierce. In a previous gig I tried desperately to hire a particular woman who had just graduated from MIT. We lost a pretty serious bidding war, not because she was a woman but because her skills were off the charts.
It's funny, there are similar articles written about certain disciplines in academia (like accounting). There too it's just a numbers game. My wife, until her latest birthday, was just 1 of 5 female accounting PhD's in the whole country (including foreign born) under 30. Of course there are precious few female accounting professors, there are very few females pursuing accounting PhD's!
Of course, that means she enjoys a tremendous advantage over her peers. The pressure to promote diversity means she enjoys a better salary and lot more job security than her male counterparts.
The same is true, from my experience, for the few intelligent and talented female engineers that I've met in the valley.