In the Windows environment, system memory is a shared resource that Windows manages on behalf of all applications. For this reason, a true flat memory model is not possible in the Windows environment. When an application allocates 32-bit memory in Windows, the memory that Windows gives the application can be located anywhere in physical memory. The memory to which the selector refers is specific to the application and does not include systemwide memory locations. In other words, the selector that the application receives does not refer to linear address 0. This means that offset 400h for the selector does not point to the MS-DOS ROM BIOS data area, for example.
Windows applications do not need to address these systemwide memory locations directly, so there is no need to map these locations in the 32-bit memory objects.