It's not about what you call it. It's designation of a codified standard. You can have a 1/4"-20 tpi box thread and that isn't going to fit a 1/4"-20 Nation Coarse which is sharp V (60 degree). Threadform is just as important as size and pitch. Even if the wrong pitch "fits" because it is sloppy enough means it will wear quicker and all need replacing.
Originally Posted by Steve Smith
The minor diameter difference is because Whitworth has a steeper thread, thus the root diameter is larger because the depth of the thread is shallower off of the major diameter of the screw.