That has been explained by women not being aggressive enough in negotiations and gender bias. There is also the issue of what various job titles should be paid though. Some occupations have significantly more women or men. Most "men's jobs" are paid more than "women's jobs". There is some evidence that some of that difference is based on gender bias rather than education or skill required.
At some companies, employees for high pay, high skill, but less physical jobs are required to work at a heavy lifting position for a certain amount of time regardless of whether this teaches any skills necessary for the higher paid job. Although some women can handle the physical limits, they may be less likely than the average man, who has a biological edge with testosterone to build muscles quicker. Supervisors may also not hire them for those positions in the first place because they don't believe that they will be able to handle the lifting requirements. In a post, a few years, I got slammed for saying this but I have male aquaintances say the same thing and I have met a few women at my current workplace who applied for heavy lifting positions at my former workplace, who should have been very well qualified but were not hired. I don't think that this is an isolated phenomenon.
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