Several answers say the operating system uses some of your RAM. This is true but not the answer to your question. In Windows, the RAM used by the operating system is, somewhat bizarrely, counted as “available RAM.”

Most likely, the answer is you bought a cheapie laptop without a separate, discrete graphics chip. Instead, you have “integrated” graphics. 2GB of your RAM is set aside for the integrated graphics hardware, meaning for all functional intents and purposes you have a laptop with 6GB of RAM, not 8GB.