In the case of the contest, how well the antagonist backstory is written definitely counts, Adherennium - Maybe Writing?. However, in the case of NaNo, it's just about understanding the antagonist. If your story helped you understand him better, you won that event. I don't love my story, but do feel I got a better understanding of him from writing it, so no matter what the results, my NaNo project is a winner, even if the story isn't.