I'd wager that more often than not the answer is because they are different people, and not, as a lot of feminists would like to believe, because they are different genders.
Edited, Mar 8th 2013 4:28pm by cidbahamut
They are different people.
And when you average out all the wages of all the different people at all the different education levels, women earn about 70% of what men do (in the US) across the board.
This is fact.
You can argue about why they make less; Maybe they don't work at jobs that are worth as much. Maybe they have to take time out of their ladder climbing to have babies. Maybe they're even paid less simply because they have boobs. But to simply declare that the whole deal is made up by feminists is stupid...and sexist.