The explanation is 100% doping. My take is that the state of testing eliminates macrodoping unless and athlete is deemed too big to fail. That leaves microdoping, but in certain corrupt countries this can be done with impunity where as in the US there is still a workable enforcement system through AIU that has to be circumvented. Doping due to biological reasons has a bigger impact on women (hence the East German swim team, Ma's Army, etc.), so if say a Kenyan or Ethiopian runner is free to dope to a higher degree and comes from a doping culture, while not quite at 1990s levels, that is going to translate into the disparity. The men have a much more even playing field and it shows in the results.