When you buy a 320 GB HDD and use it on your computer, you might notice that Windows reads it as only being 298 "GB". This is because Windows uses incorrect calculations of a GB; the drive is actually 298 GiB (or 320 GB). The manufacturer didn't scam you, they actually calculated it correctly.
MiB, GiB, TiB, PiB, etc. all use base 2 calculations.
MB, GB, TB, PB, etc. all use base 10 calculations.
Windows is reporting your file sizes wrong, but everybody is so used to the wrong that it considered right.