I've talked to one expert about a similar result in a company where I worked, and the flip side to results like this is sometimes promotion velocity - women looked "overpaid" for their level relative to the men, and that was often because they had been at that level much longer than most men at the same level. If you set initial salaries fairly with respect to gender but promote men more quickly, then you end up with…
Usually it's the opposite but for the same reason: women seem underpaid but when you correct for experience on the job, they make something like 98 cents on the dollar compared to men.