this is actually fairly normal.
if you were to buy a 120GB hard drive right now, the chances are you would only have access to about 110GB, you never ever get the full amount!
As for your situation, its also possible that there is a hidden recovery partition on your computer which contains files and software in the event of a problem.
Your best bet to get an proper answer is to contact the people who sold you the laptop and ask them to clarify.
Andrew