That stone with the “wrong last name” in your family’s cemetery plot could easily be a relative of which you are unaware.
A recent visit to Kansas located the graves of a relative, his wife, and their two grown children. Buried near them was an infant with a last name I did not recognize. The infant was the child of the relative’s married daughter and had been buried in the grandparents’ cemetery plot. A few years later, the daughter and her husband left the area and are buried elsewhere.
Never neglect those stones with “wrong names” in your ancestor’s set of graves. They very well could be relatives of whom you are unaware.