The national pipe thread standards are based on the inside diameter of the pipe since that is how pipe is used. It's all about flow characteristics of the pipe. The size and pitch of the tapered thread is always going to be much larger than the inside diameter since it affects the outside diameter of the pipe only. The most common three standards are straight thread NPS, tapered thread NPT, and tapered thread for fuel NPTF or dryseal type. There are others too but they are not common and the NPTF is also not all that common either.
The thread size and pitch are the same between NPT and NPTF but the root and crest of the threads is cut differently so the taps & dies are not interchangeable. You can get away with using NPTF tooling for an NPT fit but it won't work the other way around since NPT taps & dies will not create a dry seal.
This link illustrates it better:
https://content.cpcworldwide.com/Por...20Products.pdf