Congruent triangle theory applies to wheels too you know. Hub face centerline to outermost wheel gyro section can be easily converted using bernoulis principle in some but not all applications limited to wheel width on imperial units or not.
I've always understood that it wasn't the mounting plate, just the fact that when using backspacing as a measurement, it doesn't (or does, depending on how you look at it) take into account the size of the bead, for some reason.
I could be wrong. I don't pay attention to these things, and I never will, just going to stay blissfully ignorant in my dream world where everyone uses non-retarded standards of measurement.