SAAMI (Sporting Arms and Ammunition Manufacturers' Institute, Inc -- should be SAAMII) comprises only those companies that have chosen to be members. There is no government involvement, as there is with the governments of other countries with their manufecturers of sporting arms and ammunition. Each member company has a vote and a rep in SAAMI -- the only "governing" body involved. They establish their own guide lines, standards, etc.
As I understand it, FWIW, the company that originates a cartridge is the "authority" that establishes the pressure maximum for it (presumably according to SAAMI principles) and provides the proof and reference ammunition that all the other SAAMI member companies use in their testing, etc.
If this understanding is correct, then it's easy to see how a cartridge originated by Winchester can have a higher max-pressure figure than a very similar cartridge originated by Remington -- or even a couple of similar cartridges originated by two different crews from the same company at widely separated points in time.
Another probable source of inconsistency may be the antiquated and notoriously inconsistent Nobel system of "measuring" pressures in special barrels fitted with anvils and pistons for compressing ("crushing") those little copper cylinders. If two operators could so easily get different readings with the same loads in the same pressure gun and barrel, surely two operators at two companies, using different equipment, would be more likely to get different pressures with different loads in similar cartridges.
And there are other criteria, which I know of only by hearsay and can only guess at.
