Taking a guess, 2GB are reserved for an integrated video card.
That may affect the amount of RAM "available" but the Crucial memory wizard would still accurately report the amount of RAM physically installed (assuming the sticks were not faulty).
The wizard reports 1 x 2GB and 1 x 4GB (for 6GB total). If the graphics solution has stolen... err... is "sharing" some system RAM, Windows may report less is available.