I’m inclined to agree on compounding, and tried to convince myself that here when I was chasing the possibility that issues were coming from my axes not being perpendicular. Take a look at my attempted illustration here.
I still feel some brain hurt about the angular case… but I agree they aren’t doubled in this case. The part should be at (0, 0), but with this technique, the first side is irrelevant since wherever it is, the locating geometry is machined into it. Upon flipping, if either the machine or the part aren’t really at (0, 0), that’s the error, but that’s all the error there is. Either the machine needs to be aligned to the true (0, 0) of the part, of the part needs to be moved to the current true (0, 0) of the machine.