It's weird that I've never noticed this before. Even BioWare (likely unwittingly) adheres to this trope with Dragon Age and Mass Effect and the first Knights of the Old Republic. Though now that I mentally dissect what's going on with them, at least recently, I can see it somewhat has to do with the class systems. You generally have rogue, fighter, mage, and you get girl examples of each. For some reason it doesn't matter if you get male duplicates of any. Is there a social/psychological construct where it sticks out more if "Whoops, we made 2 female mages on this team." Like we see the guys as just guys and have as many of whatever, their abilities don't matter we just need them in the story or on the team. Whereas girls are tied to their class and are more noticeable from a mechanical view? It's interesting...