The difference between brands of primers. Well within the safety margin. Seriously, if you are worried about a 1800 PSI differential, you have a wall hanger, not a rifle.
OK, but that rifle was built for 7X57, which has a max pressure of 56,565 psi.
So do you want to use ammo that is ~4000PSI (~7%) over the design max, or ~5500PSI (~10%) over the design max?
Me, I would load down to the ~50Kpsi range. Still plenty to kill just about anything in North America.