EDO/SDRAM/DDR1/DDR2
Voltage & timings plays a very important role here across "all" types of memory modules. In a Dual Channel configuration sometimes one memory module out of the two requires 0.10v higher than the other.
Here this is considered as a fault, but is not a real fault, it just one of the two memory modules require 0.10v extra voltage to operate correctly, but this drives up overall voltage, when clearly one memory module can operate at a lower voltage than the other. So here if one of the two memory modules requires an extra 0.10v, it is automatically rejected by default.
Here all types of high performance memory modules voltage is kept to a minimum and some will fall back into the JEDEC standard with the same or better performance (speed).
For Simms, its the timing that is critical & voltage plays a less important role.