However, women have to fight an in-built discrimination. While men are more forced into work because our gender role expects it of us.
Thus, women who are candidates for promotion are likely to be more motivated and more ambitious than their male counterparts - they've had to fight harder against more obstacles to get to that point.
I see this in the gender balance of programmers. Female programmers have had to fight against a constant discouragement from pretty much everyone in their lives, while male programmers have had nothing but encouragement. So it's not surprising that female programmers tend (on average) to be better; they're more motivated, they really want this, and that's usually because they're good at it (if they weren't they'd have given up earlier).