I am not sure - all I know is that it's unfair, isn't it?

From the numbers I have seen women do not get paid less than men. Women make employment choices that are different than men. Women take more personal days for whatever reason. I saw an interesting sight one day. There were some spools of wire that needed to be moved upstairs. Two young healthy men put a rod through the spool and each taking a side walked up the stairs. Along came 4 young healthy women. They too put a rod through the spool but they lacked the upper body strength so the four of them moved the same spool up the stairs. So which is more fair to pay the girls 1/2 of what the men did since they did 1/2 the work or pay them both the same and punish the guys for being efficient?

Some of it has to do with ladies not demanding the same amount for the same work, but it's why I sometimes hire them to do various things like tutor me in Japanese or you know, Clean.

Because if I tried to get a man to do that work he'd demand far more for it.

plus i'm a feminist so I feel good about giving women work.

not a simple answer - which country are you in - are you comparing the exact same work etc?
for the exact same work and experience I think in the UK paying less to women than men under those circumstances is illegal and has been since the 1970s.

So many reasons

Complains
Personal issues
Efforts comparison
Women choose to work in low-paid roles and sectors
Women prefer not to accept hard-core jobs

ETC...