Probably because of the hair thingies. On another note, Atronachs are more fundamentally elementals than golems, so I'm removing the tag.
Note: atronachs are elementals in Morrowind and later (including Skyrim, so your actions are correct), but in Daggerfall and Battlespire, "atronach" referred to golems, and what are now called "atronachs" were called "${ELEMENT} daedra"