It's true that the average shooter can easily be the biggest variable but its not that hard to out-shoot a lot of common, every day rifles.
For instance, I know that I can shoot at least around 1/2 MOA at 100 with a scoped rifle pretty regularly. I've shot quite a few rifles that I couldn't get under an inch. Could be ammo, probably is to some extent, but its not me. I could put the gun down and pick up my .204 and put 3 in a 1/2 inch.
The better the gun, the more likely that the shooter is the variable. A really good gun in a machine rest could shoot 1" groups at 500 yards or more. Very few shooters could do double that size with the same exact gun/ammo.