Because people live in a busy world, where knowledge is not transferred with enough love and integrity. And also people are afraid to say "I don't know" and what little they know they tend not to share.
To make things more specific, those labs had uncertainty budget with something like 20 terms for the things they measured. Each of those terms had associated probability distribution etc. They had uncertainty budgets for all the methods they did etc., and some of those where probably dated, done by someone else, etc. etc. Who checks that? Is the check rigorous enough? Are some assumptions made that don't hold to scrutiny?
So it is actually very easy for error to creep in, I would say actually very likely.
To make things more specific, those labs had uncertainty budget with something like 20 terms for the things they measured. Each of those terms had associated probability distribution etc. They had uncertainty budgets for all the methods they did etc., and some of those where probably dated, done by someone else, etc. etc. Who checks that? Is the check rigorous enough? Are some assumptions made that don't hold to scrutiny?
So it is actually very easy for error to creep in, I would say actually very likely.