This issue with physical units, and all units in CSS for that matter, is that they deal with legacy code on edge case devices. Because designers use inches on sites then want to project the site on a large wall, that would directly defy the spec if one CSS inch == one real world inch. Either that or the website would appear really tiny in the middle of a massive screen.

Today, the cruft is starting to show with what were edge cases of small, large, high resolution screens becoming the norm. The specs are confusing designers more than anything else. It is unfortunate that manufactures like Apple and Amazon are ignoring suggested pixel scaling which is the driving force behind this confusion.