I heard from a friend that the Dragon armors actually fare much better against dragons than Daedric armor would. Though Daedric armor has a higher armor rating when both are upgraded there could be a logical explanation for something like that. Dragons and Deadra are ancient entities, yet a dragon could just as easily pierce through their armor considering their armor is made for another plane. Dragon armor vs. a dragon would be far more logical than using demon armor against an ancient beast of the sky. My point being that the reason why Dragon armor is on the top tier is because it is made to combat dragons and survive through Skyrim. Daedric armor is great an all, but you're better off questing through Oblivion in it than you would be to do so through out Skyrim.