Matt Parker says it's because that's how humans are naturally inclined to think, and used the midway point between 1 and 9 to illustrate. We'd say five but "children and others not exposed to math would say 3" and then gave some explanation with beads or coins. It didn't make sense to me but I do know that if a graph is log scale I need to actually look at it harder to make sure they're not trying to pull a fast one on us here folks.