In the case of the contest, how well the antagonist backstory is written definitely counts, Snowone Knows. 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.