There was a bit of a "redefinition" when digital typesetting came along. An em used to be an M-width (and only made sense as a width measurement, as it changed

There was a bit of a "redefinition" when digital typesetting came along. An em used to be an M-width (and only made sense as a width measurement, as it changed drastically depending on the letterface used at a given font size). But, then, there used to more than 72 points to the inch too.