Neither had I before you got me to think about them. Here's another thought. Whoever paid for the marker gave the stonecutter and order to fill. I would guess that the stonecutter took a work order from the customer and did the work as soon as he could. Then as now, mistakes -- even on tombstones -- could be made.
I guess the point is that nobody in the family had access to documents such as we have today, so the mistake had nothing to do with misreading a record.