Holy cow, I wrote a book!
Although the International Electronic Commission established the
term kibibyte for 1024 bytes, with the abbreviation KiB,
Windows Explorer continues to use the abbreviation KB.
Why doesn't Explorer get with the program?
Because nobody else is on the program either.
If you look around you, you'll find that nobody (to within experimental error)
uses the terms kibibyte and KiB.
When you buy computer memory, the amount is specified in megabytes
and gigabytes, not mebibytes and gibibytes.
The storage capacity printed on your blank CD is indicated in
Every document on the Internet
(to within experimental error)
which talks about memory and storage
uses the terms kilobyte/KB, megabyte/MB, gigabyte/GB, etc.
You have to go out of your way
to find people who use
the terms kibibyte/KiB, mebibyte/MiB, gibibyte/GiB, etc.
In other words, the entire computing industry has ignored
the guidance of the IEC.
Explorer is just following existing practice.
Everybody (to within experimental error)
refers to 1024 bytes as a kilobyte, not a kibibyte.
If Explorer were to switch to the term kibibyte,
it would merely be
showing users information in a form they cannot understand,
and for what purpose?
So you can feel superior because you know what
that term means and other people don't.
For an explanation of other storage units, you can consult
this helpful chart from xkcd.