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.
So it's Friday December the fifth, 80 AD, 5:30pm, and Mozes is hacking away on his clay tables to nail down the final tally of this week's adminstration of the amount of cows his boss owns and he goes "Mother fucker! These romans again ripped me off, sold me a clay tablet that only allows me to count to 720 cows, not the 751 I got! FUCK! Now I need another tablet and start tallying from the beginning, you mother fuckers!
For centuries. Justification: Jesus was dead for three days: from Friday afternoon (3pm?), day 1, through Saturday, day 2, and into Sunday early morning (6am?), day 3. Total elapsed time 39 hours. Digital computers were around last century (19xx) and this century (20xx), which is two centuries by the same logic. Also two millenia, but I find "centuries" a more satisfying word. Colossus went into operation in 1943, so that's 80 years elapsed time.