Depends on the gun . 2 identical guns can act completely different with the same ammo.
I think it is a matter of cumulative tolerances in the boltface, and chamber, each component is machined seperately and thrown into a box, later they are pulled randomly from the box for assembly.
Either that or some guns are just female. 
+1 What he said. To elaborate on it, when you make guns all day on the same machine your tooling wears as you cut each part. Make 10 parts and all the holes you put in part number 10 will be smaller than they were in part number one because the tool is smaller. Apply that theory to each of the many tools used to make each part, and mutltiply it by how many parts are in each gun. Then, just like Tom says, you throw all the parts into separate bins, and assemble those parts randomly.

On top that, as your tooling gets dull it leaves different finishes on each part cut. Once a tool wears to some specified tolerance you swap it out with a brand new tool and start making big clean holes all over again. THEN consider how much mechanical slop is in the machine making all these parts all day every day.

To save manufacturing costs mass manufacturing almost
always overuses tooling.

Employees usually get bonuses in most plants based on how much money they can save on tooling and downtime from changing it.
Guess what that leads to.

You'll never know what you've got till you shoot it. No two could possibly be the same.