The reasoning I heard is that it's explained by differences in personality across the two populations. One group is, on average, more assertive, and as a result more likely to negotiate higher salary.
Which is reasoning that's difficult to prove. To the contrary women and underprivileged minorities also feel like they can't be assertive without being labeled as shrill.
The wage gap, at firms without a history of discrimination, is almost entirely determined by women having their first child and the support structures around it (subsidized childcare, paternity leave, flexible hours).[1][2] This suggests the assertiveness is probably not the issue.
