positional resolution

Hi

A leadscrew with 20tpi and 200step/rev should have a positional
resolution of 1/(20x200) = 0.00025 in.

Have I missed something in the above calculation because I read on
the website of a system vendor listing the positional resolution as
0.000125 in.

Further, if in the above example, one uses 10x microstepping, is it
presumptous to claim a positional resolution of 0.000025in?

Many thanks

Peter