Lord's Blade Ciaran's mask has only one true eye-hole- the other two eyes are merely decorative.
The painting Guardians masks also have only one eye-hole. The Silver and Black Knights have long necks and their helms have open faces splitting from the center. Artorias' helm has an open face in the center. All of these characters physiologically are different from the gods in that they have long slender necks and thin heads- and none of them have closed helms unless there's an eye-hole near the center of the mask. befitting of a species with only one eye. Ornstein's helmet's only form of vision to be anatomically correct would be for him to see out of the mouth of the lionhead- otherwise there's no way for him to actually see out of that thing- human head or not.
Another thing of note- I'm pretty sure all the painting guardians are female. Which would lead me to believe that Male Cyclopse are larger than female cyclopse. Thus explaining the size difference between Artorias and Ciaran.