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I don't really know what purpose this mirroring would serve other than a possible attempt to remove the Motherboard ROMs from the UMA in a future platform.

It's actually pretty simple. If you open the 80286 datasheet you may find the following phrase there: After RESET becomes inactive and an internal processing interval elapses, the 80286 begins execution in real address mode with the instruction at physical location FFFFF0(H) (page 13 in my copy).

80286 and 80386 (and, most likely, all subsequent CPUs) actually start not in real mode, but in UNreal mode (since real mode address can not point to location FFFFF0).

But IBM PC software directly accesses certain well-known location in BIOS ROM. The simplest way to solve that dilemma is to put ROM in memory mapping in two places. Like IBM PC/AT did.