Pretty common to obtain a tad of erosion pitting from a loose primer, which will leave a ring the same diameter as the primer on the bolt face, or some gas erosion if the primer pierces at the firing pin hole.
I'm surprised you haven't got some erosion markings on any of your bolts Roy? What with superglue employed to glue primers in place in loosened primer pockets.
Pull out a magnifying glass and I wouldn't be surprised if you saw a ghost ring on the bolt face. Reloading cases with enlarged/loose primer pockets is a good way to get a little puff of gas against the bolt face.
My guess is the description of pitting may be overkill. What I usually see is much more minor etching. Enough to remove the gunsmith's polish on the face of the bolt. And enough to feel the roughness with the point of a pin or needle. But no pitting exceeding what I would guess to be 0.001-0.002".
It is possible to create pits, or even torch cut the face of a bolt. But that takes some real trying. Like the time a high school fella I knew was using Lee reloading volumetric powder measuring cups to measure powder charges, but didn't understand that the volumetric cups were paired to specific gun powders and cartridges. He started mixing and matching different powders (he was also reloading some pistol rounds) using the same Lee measuring cup and wondered one afternoon why his Rem 700 (.243 Win) Varmint-style rifle bullets wouldn't hit the railroad signs he was shooting at along the tracks. He mentioned several consecutive blue streaks of smoke traveling towards the targets but no hits. Then the bolt locked up tight on him. A gunsmith had to remove the barrel to get the bolt free from the action. The gunsmith told him his hand loads had welded the action to the receiver. Recommended the fella buy and read a reloading manual.
Hard to say what pressures he was pushing, but he smoked several bullets before the rear of one of his cases let loose and the gases welded the action closed. A true wild-catter!
But I agree this pitting/etching shouldn't be occurring with factory ammo in a factory rifle.