When I was an "apprentice" I was told that the meters did not read the "high leg", in the "A-base " meter trims (before socket meters), seeing as how the "high" leg has to be used with either A, B, or A+B. Having the high leg on C in the meter trim just became convention then. Something else to note is how FPE (dirt word I know) handled high leg delta in a single phase panel at one time....
Once upon a time had a boss who knew that B wasnt metered and tapped the office space heater and window AC off of it. Aside from the breaker not being listed, I couldnt handle the lack of ethics in doing that....... I didnt stay there long!