Because 32 OSs never see the whole of 4GB.
In a 32-bit Vista system there is only 4GB of available addresses. Quite a surprising part at the top of the address space is used up be various parts of the hardware. Also, I presume your laptop hasan integrated graphics controller, which alse "steals some RAM for it's own use.
Add these two effects together, and you lose up to about 1GB - varies from system to system and hardware configurartion to hardware configuration.
If you used 64-bit Vista, the address space limitation would be removed (if your hardware allowed it - but you would still lose some RAM to the graphics.