I think it may be more of a matter of how we choose to express ourselves. As has been stated, there seem to be a good number more female fanfic authors, and there also seem to be a good number more fanatical female cosplayers (about which I'm definitely not going to complain...
) than male ones, that put amounts of time and effort into costumes that make the effort most of us put into vids look like child's play. I would also not be surprised that there is a higher proportion of good female editors to bad than there are good male editors to bad... I can't offhand think of any really bad videos I've seen that I know were made by women.
What's somewhat interesting is that this isn't just limited to AMVs... my wife works in television, and while the vast majority of the employees where she works are women (if the NYC school district cancels school, they pretty much close the offices), the majority of the A/V technical people and editors are men. And this is despite the fact that I believe a majority of the Comm. majors in my wife's department at school, where she learned editing, were female.
Because I'm getting away with confusing the sexes 
.