It seems to me there must be two components to the pay gap: women not reaching the same levels of seniority compared with men with similar education or experience, and women not being paid equally with men at the same level of seniority.
Furthermore, it seems there are probably some legitimate, or at least unintentional, reasons for the former cause–such as differences in gender expectations of career ambition–and some malignant reasons–such as discrimination against women during competitive promotions–whereas there are no benign reasons for the latter cause.
Understanding the different components of these metrics would help us target the changes necessary to eliminate gender inequality, and help us understand which efforts are being effective.