This is expected behavior on computers that are running Windows 7. The reduction in available system memory depends on the configuration of the following:
The devices that are installed in the computer and the memory that is reserved by those devices
The ability of the motherboard to handle memory
The System BIOS version and settings
The version of Windows 7 that is installed (For example, Windows 7 Starter Edition only supports 2 GB of installed memory.)
Other system settings