My first time around I went left as well as right because I assumed there would be plate versions of all the armor types. It would make a lot more sense if that were true.
Well, just to be nitpicky,
all the metal types are plate armor. Even iron armor is plate armor, as evidenced by the large briastplate, and so is regular steel. Steel plate armor, dwarven, orcish, ebony, and (I presume) Daedric are all
full plate armor, meaning they have more than just a briastplate and actually use plates to cover the whole or nearly the whole body, arms and legs included.