You would expect most inmates, even in a retributive system, to not have done anything worthy of a max-length sentence. That most sentences are not two decades long doesn’t really mean much. You’d expect that in any system short of North Korea.

The US is way out of the norm for its prison lengths and number of people imprisoned, because people like the retributive feel of long sentences. The >20 year sentences are entirely pointless, but the sentences below that are frequently for crimes that would never warrant such a long sentence elsewhere. Norway, which has a maximum sentence of 21 years, has an average incarceration length of 8 months. In the United States, the average incarceration is 63 months (5 years).