Why does SanDisk Cruzer Mini USB Device 256 MB only has 244 MB

[yongqli]

I have a SanDisk Cruzer Mini USB Device (256 MB) but it shows up as only having 244 MB. Why is this? At first I thought hat by 1MB SanDisk meant 256 * 1000 * 1000 bytes but windows explorer says that the device has 256,598,016 bytes which invalidates that theroy. Can anyone explain this? Is this USB drive defective?