Well, if women really make 87% of what men make for the same work (or whatever the latest number is), it could save the company 13% or more on car insur- I mean, labor costs.
In answer to the rest of your comment, it's anybody's guess and nobody agrees why. Maybe the stat is wrong, maybe the sexism is just that rampant, maybe the women-only candidate pool is inadequate (small), maybe there is a performance difference, etc.