I can see people putting something into pin-ups to get more views but I kinda doubt they're doing that with fantasy. I imagine they're putting it there because to their mind, it is fantasy. You could perhaps provide more extensive guidelines for each category--that might help. I'm gonna go out on a limb and guess that if you start moving things around there will be a huge **** storm. Multiple categories is a great idea though I can imagine images with an airplane barely visible in the background being tagged as aviation for one of the categories etc. Good luck whatever you come up with.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken