It's already been explained to you what's different: Basically, just because the pins are in the same place and are designated as the same thing DOES NOT mean that they are compatible. Simply put, just because the same pin name is used for the same pin in the same spot on two different generations of chips, doesn't mean they actually do the same thing, and if they do, it doesn't mean they do it the same way. Meaning, 34x chips might expect current into pin xyz to come at x volts, at the rate of y mhz, etc (I'm pulling the numbers and units out of my *** here). 36x chips might have the same pin xyz, but be built to have current come in at a volts, at b mhz, etc. Different electrical currents/signals, even if the actual pin designation/purpose is the same. SO when you stick a 36x into a 34x slot, the same pins might work completely, but most likely, some will be fed too much / too little power, or get signals at the wrong rate, and thus the entire thing doesn't work, or works in an unstable / half-assed way.