A lot of the time, I think it's also affected by the campaigns. Often, movies will decide to do the "for your consideration" ads identifying the category they feel the actor/actress will have an easier time in.
I remember reading about one case where there was a supporting actress comapaign for a relatively unknown before the film came out, and then it garnered a huge amount of notice/praise and the producers jump-shifted to a best actress campaign. I suppose it would be better if I could remember who I'm talking about, but I think she didn't end up getting either nom.

Mazer31