How does PAM.GUARD prove authenticity?

[HiBoost]

I've seen countless posts referencing PAM.GUARD as the way to know if your watch is real or not. It seems to be taken as fact that if a watch registers, then it is real. I'm trying to convince myself of this but I'm tripped up on a few scenarios.

If a counterfeiter produces a copy of a real watch, then that fake will have the same model #/serial # combination as a real watch. How would the service know which is which? If it only allowed the original owner to register (or, more precisely, if it only allowed a single registration for a given model/serial pairing) then people buying 2nd hand could be out of luck. If it allows multiple registrations on the same serial/model combo, then how does it weed out the fakes?

I thought perhaps the papers, linking this model/serial combo to a particular AD (and ideally date of sale) might help make this much more robust. But then I was surprised to see people claiming that they were able to successfully register their newly acquired pieces which did not come with box or papers. So that gets me back to square one. If the registration service only goes off of data on the watch itself, how does this prove anything?