KB is a JEDEC thing where they invented a new shorthand prefix but re-used the Si prefix names of (kilo, mega, giga) while changing their meanings. They used capital K for kilo meaning 1024.

It's certainly one of the dumbest standards in existence and the fact Microsoft went with it and extended it is frustrating too.

This is the first time I hear about "metric mega", and I hang out a lot here, where Americans use their units without explanation.