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This is what I'm talking about. Outside dimensions of pipe diameters have always been the controlled dimension since the Briggs system standardized things in the early industrial revolution, and pipes of different schedules in the same nominal dimension all have the same OD with the wall thickness resulting in a different ID. The nominal designation was derived from an era where every shop had their own forging dies and practices with varying dimensions from shop to shop and batch to batch even from the same shop. Even back then the outer dimension was the controlled feature for the production process. The NPS sizes just happen to be the die dimensions that one shop was using as an internal standard and published as a compatibility thing for other shops to use during the Civil War.

This being the case, when someone grabs a tape measure and checks the inside of some unknown stock that might be pipe or might be tube I die a little inside.