The Sumerians used a sexagesimal notation system; the two lengths would be 5.10.0 and 5.50.0 (which together add up to 11.0.0, FWIW), while all of the other king lengths are 8.0.0, 10.0.0, or 12.0.0.
https://en.wikipedia.org/wiki/Sexagesimal#Babylonian_mathema...
"For example, the symbols for 1 and 60 are identical."
I did find a pattern though. If you look at ALL the kings, rather than just those 8 it becomes obvious. The recent numbers are realistic and drift further apart, becoming less and less accurate, the further back you go.
Interestingly it also compared some other clay tablets and they were close. So it seems that they were probably written down based on a verbal history that had already drifted wildly well before it was written down .
Also, one of those early kings was also literally a well known god. So it's possible they were just saying "Before the flood Gods ruled on earth", and we've since forgotten most of those gods.