Is there a reason that females only get 77% (as they like to say) of what males make?

I can only wonder how this would work...Have an all male maximum (San Quintin type) security prison with a 100% female prison staff, from Warden to cooks, yes every one. I think that would would work out just fine, at least "they" tell us that is so. I for one would not to live within 200 miles of that prison, how about you?