In doing a lot of work on findagrave, I have found that happens fairly often, particularly when a widow/widower remarries. I've found a number of times that both husband/wife of the 2nd marriage were buried with their first spouses. Makes things a little confusing for research though, so careful research is still needed. If you can find the obituary for the spouse of the second marriage, it usually gives the 2nd married name for the woman, and may indicate predeceased by spouse #1.
The purchase of the grave plot for husband and wife #1 was probably made at the death of spouse #1 or possibly before death, and it is possible that the first spouse's name was engraved at that time as well.
I doubt there is any regulation regarding the name on the gravestone. If you find the obituary or actual burial record of the cemetery or funeral home, the current name would most likely be on it.