It is not unusual. American NPT pipe sizes are based on the nominal I/D too. Tubing is generally based on O/D so 3/4" "pipe" is actually slightly larger than 1". Copper pipe uses a thinner nominal wall size so 3/4" copper pipe is the same OD as 7/8" "tube".
I have always been curious about European pipe sizes tho. I have some surplus Russian/E German gauges with a strange thread size but it is straight thread, not tapered so they must use a gasket. It is close to but not exactly 1/2" NPS (~0.84")
