I've posted about this before and I can't for the life of me find it via search. However, there's still a good, succinct explanation here. What it essentially comes down to is that the original game designers already anticipated people stacking multiple sources of armor, so they prevented runaway leaks due to the exponential nature of resistance (for example, if you have 50% resist and you add a console that gives another 10%, it's actually reducing the incoming damage you receive by 20%).