Yeah we have long standing confusion that for historical reasons KB and MB often means 2^10 bytes units so now when you see KB you really don't know what it means. Therefore I am a staunch supporter of unambiguous KiB and MiB.
I think I explained this poorly, and I appear to be mistaken about it being to save CPU cycles (though an entity such as AWS would absolutely be about saving minuscule CPU cycles that add up at scale).
I spent a lot of time researching this issue when I came across it years ago- I just remember that local file size and S3 file size was not matching up with anything and the takeaway was that S3 was calculating file size differently from Windows/Linux/macOS.
AWS uses base 2 (binary) for calculating file size, meaning 1MB is treated as 1,048,576 (2^20) bytes rather than 1,000,000 bytes (10^6).
And as you've said, one is MB while the other is technically MiB.
In case of 2^10 units the correct names are kibibyte (KiB) and mebibyte (MiB). Check https://en.wikipedia.org/wiki/Mebibyte#Multiple-byte_units
Yeah we have long standing confusion that for historical reasons KB and MB often means 2^10 bytes units so now when you see KB you really don't know what it means. Therefore I am a staunch supporter of unambiguous KiB and MiB.