First off, the spell description does not give a number for how much it increases armor. It just says "Increases armor by for 240 seconds," or something along those lines. The point being that where the number should be, there is nothing.
Second, when I cast a lower level armor spell first, such as Ebonyflesh, it gives me 330 armor (even though the spell description for Ebonyflesh says it increases my armor by 110, so I guess I'm getting a triple bonus for not wearing armor? I'm not dual casting it.) But when I cast Dragonhide AFTER Ebonyflesh, I suppose it replaces it because my armor goes up to 700ish, but in the list of active spell effects, it still lists Ebonyflesh and it does NOT list Dragonhide.
Is all of this normal? Dragonhide by itself seems to give me about 660 or so armor. Is that about right? If so, I just won't mess with the other armor spells anymore, even though it seems like a bug that they stack rather than cancel each other out.