It may be because of the interrupted cut either in machining those lug abuttments or the locking lugs or both. An interrupted cut is bad about inducing deflection in the work and the tooling. Because the bolt 'nose' is a seperate piece from the bolt body, which is really just a tube, with the bolt 'nose' silver brazed into that tube, the fit between the tube and the bolt nose many not be what it should for perfect alinement. This, taking into account, all happens at production speeds and there is much room in tolerance so these parts all fit together without any hastle. The thin section at the lower side of the ejection port is so different in mass, quenching a receiver would certainly warp it, much more than we see now. What we are seeing may come from the stress relieving process where the finish machined receiver is heated to about 300-350 deg., completely thru, and then allowed to cool without quenching, much like the custom rifle barrel makers do their barrels. I've bone charcoal color case hardened Mauser '98s. A snug fitting mandrel is inserted thru the bolt bore and a 1/4" to 3/8" thick plate is bolted to mandrel, the plate on the bottom side of the flat bottom receiver. If the receiver isn't 'blocked' in this manner, or another similar manner, the receiver will most certainly deflect when quenched and, it may deflect so much the bolt will not go back into the receiver. My 'blocking' usually will make two heat treating cycles before it has deflected enough it is not useable. I may be completely wrong about the way Remington does their receivers, machine and then heat treat and temper, but doing it that way wouldn't be conductive to keeping anything anywhere near as straight as they are now. Cutting a 700 to face the reciver, square the locking lug abuttments, or single point cutting the tenon threads for squareness is not hard on the carbide cutting tools I use, at all. The more ridgid set-up and massive amounts of coolant (and better tooling, probably) that production machines have would make it all feasible, even at production speeds and feeds.