The only thing I can think of, is that when they went to 12 volts, it would have added confusion for the normal driver if they didn't call the parts 12 volts. Nobody wants yesterdays technology. Little did they know. You can take a 1973 C10 and put that 1950 6 volt coil on it and it will run exactly the same thanks to the ballast resistor that transfers power to heat. This is the reason I went to the trouble of researching and writing this:
http://www.devestechnet.com/Home/Native12VUpgradeAs usual I didn't do this on my own. Lots of contributors in the know.
To answer your question, the ohm value of the coil (any coil) is why the 12v system see's 5.94 volts at the points. The 6 volt system is less than that at the points. Has to do with twice the pressure (voltage) and how it behaves differently in the circuit. Ballast Resistor to 8.35 volts, coil ohm value to 5.94. With 6v system, 7.8 in, 4.87 out.
You can just plug in the numbers. Not to say that the quality of an old 6 volt coil is the same as a new 12v coil, but mathematically they are the same. Oil soaked paper between the wires vs the more modern epoxy, etc. But there are no pressure differences. The "12v coil" will last the same amount of time more or less.
All edumacation aside, best to just order parts for a 56-62 engine so there is no confusion and you get the freshest parts.
