I don't think anyone is saying that all American factory shells will reach SAAMI max service pressure. That's simply a ceiling. The problem is, American ammo makers load to achieve constant velocity, using bulk powders. Therefore, while one batch of American factory ammo may be well under SAAMI max service pressure, the next batch may not be, because they've changed powders.

A careful reloader has a much more exact idea about the pressure of the loads he's working up than you do if you run out and buy a box of factory shells. All you know with the factory ammo is that it's within SAAMI parameters.