It's not normal, they should park perfectly, after a set sweep.
I would pull up the arms(wipers off glass) and operate a few times to see where they settle, they should return to the same position all the time.
If not it suggests the park switch inside unit has failed, getting a bit sticky etc.
You can still get these motors new, I bought one last month for £13.50 from ebay, brand new never been fitted.
tabetha