Nothing wrong with that. Afterall, it's pretty easy to bias just about any "research" project to deliver a desired conclusion. I think it was Twain that said there's lies, damn lies and statistics. Why so many people are willing to dismiss what they observe for themselves in everyday experiences just because some agenda driven Harvard egghead says something contrary is something I'll never understand. The next female math wiz I encounter will be the first. Just sayin.
There are exceptions. My daughter is better at math than any of us but she also has the highest IQ by far. My son needed tutors to get through some of the advanced math to earn his engineering degree, but he got there. And of the two he is the one with the better vision to see how things work together and/or envision a device to make something work.
It is understandable that women drawn to advanced math classes will have high aptitude in math. It is also understandable that most women do not have high aptitude in advanced math and are not drawn to advanced math classes. For a study to assume that women in such classes excel as well as men is probably valid. It does not extrapolate into most women being as good as men in advanced math, however.
You can go down the line dealing with all other subjects as well and will probably find that more men than women excel in some; more women than men excel in others. There is absolutely nothing wrong with that which I can see.