It's not that complex. I've seen it first hand. Here's what happens - a woman decides she wants to have children and gets pregnant. During the pregnancy her work output drops. This results in lower scores (and lower raises if relevant).
After have the child, she takes 3 months off from work. Technically, she's still employed but in reality her contribution is zero. This impacts rewards for work done.
Then after having the child the woman's attention is split between work and home and she doesn't maintain the same mindless zeal for her work as before. This means she stops staying late, extra networking, etc. This means lower bonuses, less chance for promotion, etc.
Note that the woman never left the workforce during this whole experience. So on paper she's worked just as long as her male counterpart...in reality she's stopped working just as hard as her male counterpart.
That's not to say some real injustices aren't out there but most of them are reflections of choices made within the workplace that raw salary figures don't capture.
this is basically what I assume those numbers came from. nobody wants to be the guy working the long hours, but they sure want the same pay. God forbid they get the same standards though.