My guess is that Apple did not build the memory controller with enough RAS lines to control multiple SIMM sockets. So they translate the RAS lines and upper addresses into multiple RAS lines in the CPLD and use that to support multiple SIMM socket banks.

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Which means the total memory limit for all three x100 models are the same, right?

Edit: unfortuately not, because the fact that the 7100 have four SIMM sockets means that some RAS lines are wasted, cutting max RAM by half.

Now what about the 6200/6300/5200/5300 Power Macs? If these have the same memory controller as the Quadra 630, these should support up to 256 MB of RAM, right?