This is because your computer does not have a dedicated graphics card.
You are using a integrated graphics card built on motherboard which shares memory from the RAM installed.
They can share from 1 MB to 256 MB.
If your computer shows around 1-200 MB less than the RAM installed - its okay, anything more your RAM is not installed properly / working.
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Comment #1
(Posted by an unknown user) Rating
Also if you're using a 32bit OS (Vista XP etc)and have 4GB or RAM installed it will not show all 4GB...it's a 32bit thing.