My theory is that people who are closer to their rural roots tend to put the paper down the back.
When TP was invented in the 1890s, people in the outback were mostly using outhouses. Those sheds are small. If you put the roll with the paper on the outside, you will have paper in your lap. So instead you put it so that the paper goes down the side of the wall.
When those homes later installed inside flush toilets, people kept their old habits, running the paper along the wall.