Why don't the ATR 42's propellers start/stop rotating simultaneously? Why is it that for the ATR 42 one of the propellers begins to rotate before of the other?

I've noticed that they also stop rotating at different times. Is there a reason for that or is it just for turning left or right?
Pilot only starts one engine at a time so the battery doesn't have to spin up both at once. At turnoff, one stop button is pressed before the other.