Most laptops have integrated video cards that use a portion of your system memory as memory for the video card. Your particular laptop is available with an integrated or discrete video adapter, so you'll have to check the specifications of your precise build to be certain. However, there are some settings in Windows or your BIOS that could cause this.
According to Microsoft, the "usable memory" calculation on the System control panel is inaccurate, so take whatever it says with a grain of salt. You can double-check whether this memory is actually being reserved by hardware using the Resource Monitor application included with Windows 7.
There is more information on this calculation, the BIOS and Windows settings you can change that might be affecting it, and the Resource Monitor application in this article from the Microsoft Knowledge Base.