The answer to that question is that all information on a tombstone is from a secondary source.
The first reason that a tombstone is considered a secondary source is that the information on the tombstone is only as accurate as the informats memory. It is so easy for birth dates and places to be wrong - even for a name to be wrong. One of my ancestors came to the U.S from Germany with his birth name being Anton Keppler. But on his tombstone, he is listed as Anthony Keppler, because as he assimilated with American culture, he let his name become Americanized.
Sometimes, it took a while for the family to be able to afford a tombstone. So by the time the information was even put on the tombstone, it could be weeks, months, even years later. During the Great Depression, it was especially common for families to not be able to afford a tombstone. Money was simply not available for a tombstone, and sometimes families had to wait years before they could afford one. All of the information is now left up to family members as they are forced to try to remember dates.
Another reason why tombstones are considered a secondary source is that sometimes the stonemason made a mistake when carving in the details. Sometimes a name is mispelled or a date written incorrectly. You would think that these stones would be replaced, but sometimes the money was just not there.
The moral of the story is to never assume that the information on the stone is correct - more research is needed to confirm.
