It is the difference between what the manufacturer defines as a mb and what windows defines as a mb..
The manufactures say that 1 kb is 1000bytes. However, windows defines 1 kb as 1024bytes. So there is a small difference here.. but the bigger the drive size the more you "loose" to the math.
http://www.wired.com/news/business/0,1367,60505,00.html