OMG I FINALLY GET IT
I always thought the "under" people simply didn't understand the basic concept of TP. "Over" is how it should be, so the first sheet is easy to find and the wad you need is easy to tear off, right?
Then I thought about Emma's brief dabbling in TP unrolling, and "under" now makes perfect sense. Even the most obsessive cat can't easily unspool an entire roll when it's facing the other direction.
::mind blown::