[Klaus]

Because it is not as crystal clear as you suggest it is. :-)

Nano USM is a USM variant - using a ceramic piezoelectric element just like Ring USM - that doesn't drive a rotary action but a linear one based on a guide bar system similar to STM.

Thus the wording is hardly wrong but just not overly precise. ;-)
That being said - I don't care too much really. It's the result that counts and the result is superb.