A couple of things contribute to that. The left side of the lane is used less and if oiled properly at the beginning of the day will hold up better so it is more consistent and slides better. Secondly if the balls are stock ones from the bowling alley they are drilled in the middle and not offset or weighted. To get spin you have to use wrist action. But if the ball is his own and customized it will often be weighted and offset specifically so it will spin naturally. Some folks even have a different ball if they want to throw flatter and with less spin like when they are making spares.
You know the game.... And no I am not being sarcastic.