"Units can have different historical values. A foot is defined as exactly 0.3048 meters. Before 1959, though, it was 0.3048006 meters. This is means that all historical documents have a different measurement of foot. And there was a period of time where we used both definitions in different places!"

In the US, different definitions of a foot are still used in different states today. The plan is to standardize on 0.3048 meters at the end of 2022.

https://www.sco.wisc.edu/2019/10/18/us-survey-foot-to-be-dep...