The details put on gravestones are notoriously unreliable. It's not unusual for families to get things wrong, at a time of emotional strain, especially if some time passes after the death before the stone is chosen, or before additional words are added to an existing stone. If there are different members of the family involved in registering the death and arranging for the wording on the stone to be completed, even a different date of death may be the result.
It's like Chinese Whispers, that game where each person in a line whispers a message to the next one along. Everyone agrees to the stone, but one person is left to pass on the exact wording to the Monumental Masons who deal with the gravestone. Perhaps he or she is in a hurry/ has handwriting which is not clear / loses the piece of paper intended to be left with the masons and has to supply the info by word of mouth just relying on memory. Just some suggestions as to how things can go wrong. It's not unusual.
