At first glance, this doesn't look so strange. It's an ancient stone tablet from around five thousand years ago, upon which the reign of every ancient Sumer king is recorded. It's only when you look at the oldest entries that things become strange.
According to the text on the tablet, some of the prehistoric kings ruled for centuries. Two or three of them are credited with reigns that lasted thousands of years. A mathematical error seems unlikely because the rest of the record is accurate, so what's going on here?