OP, your head may be exploding, but take a second to notice something in the pictures you posted. Do you notice that the comparison is not showing that you will gain or losing armor rating (like it does the other stats in the first pic) by equipping the item? I believe the problem is that the tooltip isn't updating the true armor rating of the unequipped piece, even though the comparison tooltip (and the game) is recognizing them as equal armor rating.