I don't know much about AVN awards to say the least but from a quick bit of Googling it seems the best ass award is something that is voted for by fans and has been won by her every year it has been going... which is only about 2 or 3 years I think.
And I guess if people like her ass that much, opinions won't change much from year to year.
Dunno if there is any "minimum number of scenes shot during year" criteria or even if people vote from a shortlist of nominees or whether they have the freedom to vote for whoever they like.