There are three rolls of TP in the stall, each with different amounts left. The two with the most left are on dispensers / roll-holders, and the one with the least paper remaining is on top, meaning that if you used the roll on the dispenser underneath, the one on top would will fall on the floor. Which roll do you use?
[My answer - the ones on the dispensers are there to be used. The one that's not on a dispenser is a spare. Use the smallest of the ones on dispensers, so that the larger one will still be as close to full as possible when the smaller dispenser-roll runs out.]
edit: oops, forgot to answer the orig Q - over. Maybe it's slower when it's under but I will waste invest the time it takes to dismatle the dispenser, put the roll the right way around, and put it back together again if I need to.