Why is the system memory reporting in System Information less than 4GB with 4GB or more installed?

Why is the system memory reporting in System Information less than 4GB with 4GB or more installed?

This is a limitation of the 32 bit Operating System.
If a computer has 4 gigabytes (GB) of random-access memory (RAM) installed, the system memory  that is reported in the System Information dialog box is less than expected.
For example, the System Information dialog box may report 3,120 megabytes (MB) of system memory on a computer that has 4 GB of memory installed (4,096 MB).
Various devices in a typical computer require memory-mapped access.
This is known as memory-mapped I/O (MMIO). For the MMIO space to be available to 32-bit operating systems, the MMIO space must reside within the first 4 GB of address space.
To avoid potential driver compatibility issues, the 32-bit version of an operating system limits the total available memory.

If a computer has many installed devices, the available memory may be reduced to 3 GB or less.
However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB.