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.
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.
[1] https://www.forbes.com/sites/lisaconn/2023/11/08/nobel-winne... [2] https://www.stlouisfed.org/publications/regional-economist/j...
I don't know if that's the case here. But it would be good to investigate all the possible factors before coming to any conclusions.