The precision and methodology of reporting changes.
As posted, more entrepreneurial lawyers are on the streets and more boomers, who have never taken any responsibility for any lapse of judgement or stupidity, are of an age to have guns and they have been taught -by the lawyers- that a suit against a large corporation is akin to winning the lottery.
Few bullet companies can agree on exactly what size a say .308 bullet should be. Some companies use test barrels and others use actual rifles for testing. Different brass; different primers; different climatic conditions, etc. All contribute to varying results. Best bet is to have several load books and review them all when starting with a new calibre. Do not consider any of the books as "absolute".

A lot of the problem traces back to the inability of either the piezo or the CUP system to reliably get the same answer over and over again, even using the same equipment and the same technician. Add in variation from lab to lab, and you have a lot of measurement error. Even averaging 10 shots, as SAAMI calls for, the CUP system is doing good to repeat itself within plus or minus 1,000 PSI. In '06 size cartridges, that's about half a grain.

Another problem is that the ANSI/SAAMI test procedure, which all the manufacturers use, is far overly rigorous in controlling the temperature of test ammunition, and almost completely fails to control chamber temperature, which has about 2-3 times as much influence on pressure and MV.

I don't discredit the "lawyer" influence on the situation, but the fundamental problem is two accepted measurement systems, neither of which is exceptionally good, and failure to control chamber temperature, which is a very strong variable.

I do know that Hodgdon uses a special batch of powder for their testing. They use a selected batch of powder that is right in the middle of the specs for that variety of powder. So half the people who load to their spec will get higher MV and pressure, and half will get less.

Because reloading manuals are only cookbooks, not bibles, and yes, components as well as firearms and testing equipment changes. In addition more lawyers have been graduated...