I've never liked the way they size these things, but for those who don't know, here's the reason for the confusion... the numbers don't really match the size although it's close on the inside diameter.
I agree; I think the NPT sizing refers to the ID of a piece of black iron pipe, which of course has nothing to do with old Mopars, unless you are installing a gas furnace or something.
The table is the only way to make sense of it.
