Off center firing pin strike can be caused by several different things. It may be the firing pin hole in the bolt as mentioned above or it frequently be the receiver threads that are not concentric with the bolt way in the action. The bolt/reciever clearance is a factor also. And the barrel threads can be eccentric with the chamber. All four of those issues can be present at the same time (stacked tolerance) in any amount. If you have pin strikes that are way off center your rifle likely has more than one of those issues. The good news is that if your rifle shoots well enough to do what you need it to do, you don't need to do a thing about it.