As mentioned above, many variables in individual processing techniques, water quality, pH, meter sensitivity, shutter speed accuracy, shutter speed efficiency, etc. cause the individual variations.(posted 8389 days ago)Then, too, is the developer used for ISO tests can't be bought, and you probably wouldn't want to use it. And, I believe, their developing technique isn't anything commonly used. The developer & technique are chosen for laboratory repeatability, something very important in developing a standard, but less so for users (not that we can be sloppy).
Note that with the TMax films, Kodak chose to give EI (exposure index) numbers instead of ISO, probably trying to give users a better sense of true usable speed than meeting a standard.
Charlie