They've been doing this for literally centuries.

I think it started out with a rare case of honest advertising. So for example 720K floppies were advertised as 720K. But then some ~~lying bastard~~ clever marketer decided to start advertising their 720K floppies as 1MB floppies, sometimes but not always marked "unformatted capacity".

And of course this had the desired effect of making people buy their disks instead of the honestly marketed ones, because people didn't read the small print and thought they were getting more storage, which was important before CDs were a thing and software distributions were starting to need multiple disks. So everyone had to start doing it.

This is as far back as my memory of the practice goes, so it may have started before 720K floppies were mainstream, but that's why disk manufacturers now advertise the unformatted capacity of their drives instead of the formatted, aka usable, capacity.