As I understand it, the categories are based on length, and I suppose there needs to be some way to distinguish the medium-shorts from the short-shorts to help even up the field of competition. Perhaps in the future they'll also do it with the novels, and distinguish epic cockroach-killer tomes* and multi-volume splits from self-contained mid-length tales.
Myself, I wasn't too impressed by most of the Novelette/Short stuff this year (though I really liked a couple of them), but the Novellas were all excellent and well worth reading (and Googling for the ones which are promotionally free to read online). I had a tough time deciding which order my votes for that category would go in.
And given the subject matter of
Ponies† and
The Things, I do kind of wonder if there's going to be a trend of retro pop-culture shout-out ones in the future.
* I, for one, welcome our Best Epic Cockroach-Killer Tome Award-winning overlords.
† This story was just begging to be a faux-cutesy illustrated girly dayglo pastel picture book.