It's doesn't really affect my gameplay but I still find it weird that it happens on my left hand and not on the other one. Does anyone know why this happens?
This is completely normal among mania players. It will change overtime, most likely. As to why it happens, I (think) it is due to the intensity of maps. So if a map gets denser, you will try to play faster in response, and I guess that means changing the position or form(?) of your hand.