Another thing is this custom: https://en.wikipedia.org/wiki/Damnatio_memoriae https://en.wikipedia.org/wiki/Damnatio_memoriae Imagine that applied on a large s

Another thing is this custom: https://en.wikipedia.org/wiki/Damnatio_memoriae https://en.wikipedia.org/wiki/Damnatio_memoriae Imagine that applied on a large scale after a revolt or war. The enemy would be so vilified a law would be passed to wipe the time they were in power from history. Then later generations try to piece together what happened and can't tell whether that "forgotten" dynasty lasted 100 or 200 years, but they do their best and guess 200 years (nope it was 100). Then we base our history off records of that earlier guesswork.