(Crosspost, but what the hell)
Also, why do they persist in selling drives with a mislabled storage (e.g. the label says 2Tb but it's 1.81Tb).
+1. I have wondered that for many years.
A couple of issues:
1) For convenience, and because 2^10 isn't far off 10^3, Computer People use the SI prefixes to denote powers of 2, so k means 1024, M means 1048576, and so on. Meanwhile the rest of the world (but notably in this context Telecoms People), use them in the traditional power of ten sense, where k means 1000, M means 1000000 and so on. This isn't usually a problem until you're trying to work out how long a file of foo gibibytes
[1] will take to transfer on a network link of bar megabits per second, or similar.
At some point the marketroids worked this out and realised that if they used the decimal prefixes in their specifications, they could make their storage devices sound bigger. All the manufacturers quickly followed suit, so as not to lose the arms race, even if just amongst their less literate customers. And now we're stuck with it.
2) Filesystems have overheads. You don't, outside of a few specialist contexts, just spew data to a storage device bit for bit. There's going to be some metadata to say where the file is (bear in mind it might be fragmented all over the disk, so this is non-trivial), what it's called, permissions and so on, as well as provide error-checking/correction. More subtly, the filesystem will divide the space up into blocks, and only store [part of] a single file in each block. Bigger blocks mean less metadata is needed, but more space wasted if the file is smaller than the block size (it's a judgement that you have to make when formatting a disk, based on what it's intended to be used for). This is how a file might appear to grow or shrink when copied from one filesystem to another, and why your file manager might quote different values for "file size" and "size on disk".
3) People sometimes confuse bits and bytes in abbreviations, because Non Computer People who aren't German aren't used to upper and lower case being an important part of the spelling. GB is gigabytes, whereas Gb is gigabits. The former being 8 times bigger, which is small enough a difference that you might not notice the way you would if you confused millilitres with megalitres.
[1] In a belated attempt to reduce confusion the IEC came up with new prefixes (kibi, mebi, gibi vs kilo mega, giga) for the binary-derived values. People sometimes use them. My recommendation would be to approach the specification of storage devices with the same degree of caution that one approaches the sizing of bicycle tyres, and to assume the decimal meaning unless it's the funny 'i' version.
