Why is the thread diameter of #6-32 stainless steel Phillips screws different?

Among our commonly used screw thread diameters, there are M3, M4, M5, etc., these with “M” in front of them refer to metric threads. In addition to metric threads, there are imperial threads in fasteners, such as #6-32, this means that the thread diameter is #6, and “32” means that there are so many threads in one inch. But if you have measured a lot of #6-32 stainless steel Phillips screws, you will find that their thread diameters are somewhat different. Why is this?

If it is a standard #6-32 stainless steel Phillips screws, their thread diameter range is 3.33mm-3.48mm, this is UNC coarse thread, and the thread angle is the thread size of 60°. At the same time, this thread size also means that the screw is produced according to the 2A tolerance code.

In addition to the tolerance code of 2A, #6-32 also has a tolerance code of 3A, and their thread diameter is 3.35mm-3.5mm. Tolerance codes are different, their thread diameters are different. Therefore, sometimes when we measure #6-32 stainless steel Phillips screws, the thread diameters are different because they use different tolerance codes.