All threads have tolerances, making them an easy fit or a precision fit. What you are seeing in the minimum and maximum sizes are the tolerance allowances. Naturally if a shaft were made to maximum size and the corresponding nut were made to the minimum size, it would be a tighter fit. The opposite applies if the shaft were made to the minimum sizes and the nut is made to the maximum size, it would be a much looser fit.
On my 5C collet block set, the ring nuts supplied are really sloppy on the collets, probably to make them easier to get on. In an application where a lot of torque pressure is required when tightening, then a more precise tolerance would be required.
Paul.