There are much better studies than this that look into the question honestly.
The issue is, if you look for cliffs notes or sound bites than more liberals will quote a number without other context (although the gap is real even with contexts) and conservatives will pretend it does not exist and make it purely about choice, which it is not.
As I said to myseteryman I believe the problem is not sexism as such, it is economical. Women of breeding age are more expensive. Becuase they go off and have kids and you still have to pay them.