Simply the motherboard revision can't be determined in software.
On other platforms where software can recognise the motherboard, because that information is stored on one of the chips e.g. the BIOS, the software might still not pick up an individual revision. A real example is that I have a PC with an Asus A8N-SLI motherboard which was replaced under warranty. The new board was ever-so slightly updated with a fan power header in a different place and so probably some other changes. But it was still an A8N-SLI and nothing reported any differences.
On an Amiga 500, the motherboard is the green circuit board containing the tracks and sockets. The other bits such as the actual chips and LEDs can be interchangeable. (I THINK that some people have even taken an A500 OCS and put in ECS chips to effectively make it a 500+). So if the information to determine a motherboard is stored in a chip, and you have swapped that chip out, the information reported back would be wrong.
