it's all down to the d20 system - the gap between 'really really bad' and 'really really good' is only 20 wide - so if the devs add mobs with AC in the 60s then mobs with ACs in the 40s become 'hit on a 1' again. The same problem affects player AC and (as someone has already said) spell pen numbers.
I posted a while back about the 'd40' system or a 'd100' system - but DDO is pretty much tied to the d20 system so -meh- what can we do?
Garth