Recently, I had two storages on hand: one was a relatively old 1TB Toshiba HDD, the other was a brand-new 1TB Samsung SSD. I have known for quite some time that the capacity labels can be a bit tricky. Out of curiosity, I ran fdisk on both devices to compare their actual capacity; it returned the exact same value for both: 1000204886016 bytes.
I expected 240, or 1012, or two distinct seemingly random values somewhere in-between (I heard that the manufacturers usually keep some part of the storage in reserve). But certainly not an unique seemingly random value (and it's not even close to an integer value in MiB). And after a short research, I found that value again on this question and that question (unix site) (and Google returned a significant amount of results).
Is this some kind of convention, or just a surprising coincidence?