Mine is only 3.75GB usable. There are other parts of you computer (your video card is one) that needs RAM, too.
What I don't understand is the users with 32 bit systems, their OS only can use 3.2 or so RAM. Why can't that remaining .75GB RAM be used towards these things, instead of pulling it off what the OS "sees"?
All the memory on your system, every single bit, must have an address. That includes cache, memory on the video card and the system RAM. These addresses are called MMIO addresses. There are 2^32 power available on 32-bit systems, or 4,294,967,296 bits, better known as 4096MB, or 4GB.
Windows reserves a flat quantity of these 4 billion addresses for non-RAM addresses, and takes more if needed. The remainder is given to system RAM at about 3.25GB.
32-bit Windows could address more memory, but tons of 32-bit programs would crash if Windows placed their data outside of the first 3.25GB of system memory.