Ammo factories come up with seating depths by shooting rifles from different makers and coming up with one depth that gets best all around by average. It's why some factory rifles like Winchester fac. ammo while others like Fed. and so on and so forth. It goes by whatever rifles they tested it with. I've got quite a few rifles of the same cartridge that each and every one of them like a different load as well as seating depth. Got a matched pair of old Colt .45's with following serial numbers that won't shoot the same loading worth a damn. No ammo maker could live up to the claim that every rifle will like their brand. It's impossible to do. And back in the "old musty days" rifles could be, and a lot of times were, made with closer tolerances than they are today. Hand crafted, closely inspected, and top shelf quality control has went the way of the Greek Gods. Simply a myth anymore.