They're binary prefixes[0]. If we go by the traditional meaning of the SI prefixes, "M" means "10^6", so 1MB is 1,000,000 bytes. 1MiB, by contrast, is 1,048,576 bytes (1024*1024, or 2^20), which would be more correct for something like RAM.
Yes, in the past the SI prefixes were "abused" for power-of-two quantities, but nowadays it's best to be more precise, as many computer-related things actually do come in power-of-ten quantities, not just power-of-two quantities. For example, my NVMe drive's capacity is actually 2TB, not 2TiB[1].
[1] Ok, ok, it's actually showing up in `parted` as 2,000,398,934,016 bytes, which is a little bit more (~380MiB) than 2TB, but considerably less (~185GiB) than 2TiB.
Yes, in the past the SI prefixes were "abused" for power-of-two quantities, but nowadays it's best to be more precise, as many computer-related things actually do come in power-of-ten quantities, not just power-of-two quantities. For example, my NVMe drive's capacity is actually 2TB, not 2TiB[1].
[0] https://en.wikipedia.org/wiki/Binary_prefix
[1] Ok, ok, it's actually showing up in `parted` as 2,000,398,934,016 bytes, which is a little bit more (~380MiB) than 2TB, but considerably less (~185GiB) than 2TiB.