I just assumed, the volts were added up for each leg that came in. EG: 240v = 0v, 120v, 120v.
I'd think if you measured one leg of the 240v with the other, it could come up a wiggly 0 volts.
But thats just what I'd think. I'd never know until I tested it, and I've never tested it.
But just one question, do AC meters measure 3 phase voltages correctly across two hot legs.
You're my AC guru!!!  Wouldn't you just love to have an oscilloscope these days.
I thought you might know all about those HOT LEGS!!! har har!