I know this a a joke but in case some people are actually curious: The manufacturer gives the capacity in Terabytes (= 1 Trillion Bytes) and the operating system probably shows it in Tebibytes (1024^4 Bytes ≈ 1.1 Trillion Bytes). So 2 Terabytes are two trillion bytes which is approximately 1.82 Tebibytes

The result of marketing pushing base 10 numbers on an archiecture that is base 2. Fundamentally is caused by the difference of 10³ (1000) vs 2¹⁰ (1024).

Actual storage size of what you will buy is Amount = initial size * (1000/1024)^n where n is the power of 10^n for the magnitude (e.g kilo = 3, mega = 6, giga = 9, tera = 12)

I’d be thrilled if the SSD I bought ended up being almost 8x larger than advertised! Does beg the question of why you’re buying 250GB SSDs in 2023 but I’m not here to judge.

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.