I suspect the DIMM slot "overlaps" a pair of the SIMM slots. So, you can have 4 SIMMs or 2 SIMMs and a DIMM. So, your 64MB DIMM, plus 16MB of the SIMMs makes 80MB. Generally this is all controlled by logic on the motherboard (these days the chipsets are standardized enough to say that functionality is in the "northbridge" chip). It decodes the high order address lines, and selects the appropriate memory socket (or device chip, for devices with memory mapped hardware). I suspect the newer DIMMs you were unable to use were PC100 (100MHz bus speed) or faster, when your motherboard wanted 66MHz or slower (I forget the flavors of SDRAM which might also affect this). I think EDO was only for SIMMs, but that could be a factor too. And, yes, the address space of the motherboard is almost never as large as the address space of the CPU.
Memory