Perhaps jobs that pay less are less attractive to men who can be better paid at some other job, so these jobs attract more women.
If the above is true, then there is indeed a "pay gap", but it is not the consequence of any systemic sexism.
So a woman in a female-dominated industry may suffer not only from their own negotiation tactics but also the negotiation tactics of most of the workers in the industry
As someone that works in the biotechnology industry, I can see a lot of corollaries. Scientists love being martyrs and there is a strong culture of "don't care about your pay - only care about the work." I could see how women could be more susceptible to this line of reasoning and/or taught to care more about "passion" than their bottom line (compared to men).
Again, lots of spitballing. Probably way off.
It could also be due to men leaving industries that have ongoing lowering of wages.