The problem arises when the OS uses GiB but labels them GB, like Windows does.

Look at Mr. "100 IQ" over here

Yeah:

1000 GB × 1000 × 1000 × 1000 / 1024 / 1024 / 1024 = 931.32 GiB

It's a sufficient explanation. I don't know why the meme has the highest IQ person say "I don't know", unless the joke is that it's easier than explaining...

Which itself is for marketing reasons, a manufacturer would find it difficult to compete with others using units that make their drives look larger or cheaper at "same capacity".

There is also the smaller amount w!ndows will secretly waste for its own pleasure (few GiB iirc). And an even smaller amount that gets wasted in partitioning. Usually around 5MiB, you can reduce it to 16kiB with some manual configuration in gdisk, or you can skip partition tables entirely for some end-game cursedness. And then the filesystem will take some. ext4 takes based on the number of inodes, for large drives that can be tens of GiB. Btrfs takes almost nothing (though in the background allocates in steps of 1GiB, so some tools might show 2GiB used as soon as data is written). Then if you try to sum all files on disk, that will land even lower for many more reasons.

At the beginning of time, K/M/G etc was based on 1024. This was considered a bad move by some people.

What I don't get is memory packages don't come in thousands, but in volumes dividable by 1024, so why the 1TB=1000 somethings...

And it gets more complicated when we get to the metadata and filesystem structure. So, you technically never get to store 931GB, I wonder what the (average) ratio of metadata to data is? 3:1?