General aggregation question

I was curious as to what might be causing an aggregation to take significantly longer when run for a different fiscal year, all other things being equal. Each fiscal year is set up identically as a member in a Years dimension and has roughly the same amount of data, but for some reason, the current fiscal year takes about 30 times as long to complete as every other year. Is that because it has something to do with it being data that's being modified currently, or are there other possibilities?