Typically it is the left side, for right handed folks, that often is more flat. From what I learned it is because your body gets in the way of rolling to the left. This is why you see a lot of turners (me included) lean to the left to complete the bead. Sometime I will practice with my left hand as everyone says it only takes about an hour for it to become "normal"." I keep ending up with a cut with a flat bevel instead of a radius. I can do it with a skew though. Go figure!