This may explain it for you:
http://articles.networktechs.com/36-p1.php
Most operating systems define a hard disk drives capacity using binary or base-2 mathematics. This translates to 1 gigabyte (GB) equal to 1,073,741,824 bytes. This is the correct value when using binary or base-2 mathematics.
However, hard disk drive manufacturers define drive sizes using base-10 mathematics, in which 1 GB is equal to 1,000,000,000 bytes (rather than the 1,073,741,824 bytes, as listed above).
This discrepancy in reporting drive sizes (base-2 vs. base-10) may lead you to believe that you have a hard disk drive of less than expected capacity if you compare the figure reported by the operating system with the figure reported by your documentation, although the actual hard drive size is identical. Microsoft Windows simply counts the size differently, and will report a different, slightly smaller, figure.