why would you sign off oil leases mineral rights you didn't think you had

The oil company is probably trying to make sure that anyone that could have received an interest is signing the interest over to them. Perhaps the probate was not completed. Perhaps there were questions about the will. Perhaps a land-man found something prior to your grandfather's receipt of the minerals that caused a question in the heirship. Perhaps they are merely being extra-cautious. If you have any questions, then you can probably ask a landman. You can see whether they will pay you a little extra money for the extra acreage, but I am unsure of their answer.