why

Below 14" (12" and below) OD was forced by the pipe thread , ID becamd incidental (thichness)

For the later versions humans became more intelligent and controlled the diameter (not ID but OD is 14") and the threads became a bit more round figure.

Just look at the dimensions required for say NPT 1" (OD 1.315") and NPT 14" (14")

1. Same as O.D., not I.D.

2. Because--just because.

3. Beyond that, it will be necessary to search into some arcane history.

4. It was ordained by God, but even He don't know why.