Cottonelle's whole "over vs. under" poll is ludicrous. There is one and only one right way to mount the toilet paper on the holder: hanging over.
A character in one of their commercials demonstrates the only permissible exception to this rule. He has a pet who unrolls the roll if it hangs over. Since Hobbes used to do this when he was a kitten, I appreciate the dilemma and consider this an acceptable compromise. But in any other situation, under is wrong, wrong, wrong.
I present the following scientific analysis to quell any doubts.