As for why people use 1000/1024 its basically marketing vs binary which is why your Hard drives never have quite as much as they're advertised to have.
Firefox also uses powers of 10 instead of powers of 2 for downloads and such, and some *nix command-line utilities even provide a switch that displays "SI" units instead of binary ones. Also, there's a way of distinguishing the two: kB vs. KiB, MB vs. MiB, GB vs. GiB, etc. The former are the SI variants (i.e. power-of-ten), the latter the binary ones. Technically, the binary terms are different, too, not just the symbolic notation: "kibibytes", "mebibytes", "gibibytes", etc., but personally I find those silly to write/speak aloud and just say/write the "SI" forms when I want to wrte the full unit names, and use binary prefixes otherwise. The main exception is if I'm talking about HDD capacity (because yes, hard disks are measured with powers of 10 for duping^Wmarketing reasons). Yes, I'm inconsistent like that, because I'd rather not cause confusion for people who aren't pedantic and/or aren't familiar with the "proper" terms for the binary units. <.<
See also this
, particularly the mouseover text.