Upper receiver lapping?

[Wheelhorse]

Upper receivers are either forged and finish machined, or machined from billet aluminum, then anodized.

The boring and face-cutting operation is done so that there should be no way for the face to be out of square....however, once anodizing is done, it can create a face that is no longer square due to non-uniformity in the anodizing process (a common headache in the anodizing process).

This is 100% fact. Anyone who doesn't believe it is just lucky to have never encountered an upper that isn't square because they do exist and the reason just given is the only thing that makes sense.
I've seen two.