It’s like the heter with showing hair at the front – a specific ruling was given by R’ Moshe for a very specific sheila and it has somehow been twisted to become a worldwide ‘accepted’ ruling.
IIRC, he doesn’t write that context into the teshuvah. However, he does acknowledge that he’s arguing with the Chasam Sofer, and worse, people somehow use it to expose a lot more hair than he ever allowed.
Tefach means area, but it seems that many women think it’s linear. And they’re makpid on the Chazon Ish’s shiur.