Restricting things to gender makes it vary obvious that it's not a single stage pipeline, because YC get's much closer to 4% women applicants than 30%. The simple fact is for a variety of reasons a lower percentage of female graduates with a CS degree are interested in moving to the valley or joining / starting a start-up. That's not to say there is no discrimination just that discrimination does not account for the massive differences by it's self.

If we really want to understand what's going on we need to look at areas that attract more women than men. It turns out that the northern Virginia area has a significant female bias where there are far more young well educated females than you would expect based on graduation rates etc. Perhaps the prospect of a safe 80+k job in a good area acts a golden handcuffs that prevents many young capable women from 'seeking their fortune'. But, good or bad, cultural or biological, it's not simply a question of employer bias, people chose the areas they want to live in far more than they are force into or out of a specific region.