It is NOT "complicated." It has to do with the fact there are two kinds of jobs in this economy: primary wage jobs and secondary wage jobs. These are based on the notion of the "family wage." The primary wage says there is a breadwinner, the man, while the secondary wage says that person is a woman, who is being supported by a man. To put it bluntly, this has NOTHING to do with women's skills, women's education level, women's alleged dropping out of the labor force for years at a time---which almost NEVER happens these days unless the women are wealthy--it has to do with the belief that women don't need the money as much as men do because they are supported by men. Men, on the other hand, need more money than women because they have families to support or will have families to support. WHY do people need to lie about this and claim it is "women's choices" or "lifestyles" when in fact it is SEXIST ideas of how women live their lives motivating the pay differential?
If this were not true, jobs that women primarily do would not be paid less than what men do in work of similar levels. Every single time an occupation is dominated by women, the pay and prestige goes down. Why? Because of the idea they don't need the money because husbands support them. This attitude is why women, especially never-married women, far so poorly in old age.
My rant is over this article "celebrating" International Women's Day by asking why women continue to earn less money than men.