Thanks, that explained it nicely! Some of the tricks built into these old systems are quite fascinating.

Anyway, the reason for my curiosity: I'm working on building my own computer on an FPGA, and I'm taking some inspiration from the older macintosh systems. I don't think I'll copy this bit of complexity though; instead I'll use the uppermost address byte to split my memory up into sections enabled by chip selects. ROM at the bottom, then RAM across a number of sections, and the remaining to be used for memory mapped peripherals.