Dragonp armor is actually superior to Deadric. Obviously, you're using the Smithing skill (and not finding Dragon armor loot after level 40 or so), and I'd venture to guess you invested in the Heavy Armor skill, too.
Most heavy armors can reach the armor cap (576?) easily with just one point in the Juggernaut perk, Dragon and Deadric among them. The difference is that Dragon armor weighs less and is more readily available, which means you can carry more stuff (including different armor sets--usually difficult for Heavy Armor users, but useful if you have different enchanted armors for different scenarios) and sell it more reliably.
If two armors provide the same protection (and by the time you can make Dragon armor, it
will provide the same protection and not single point less), then the armor that weighs less and is more readily available is obviously and inarguably superior.
Hooray for pressing the Enter button at random points. This isn't World of Warcraft, stop gawping at the stats and enjoy the game.
Also, this. Even if Dragon armor was incapable of reaching the cap, +500 armor rating is not too shabby, and certainly not bad enough to warrant all this whining people do all over the boards.