The unique helmet has the same armor rating and weight as the plain unenchanted variant, so for consistency it should use ArmorMaterialSteel (instead of ArmorMaterialDaedric) so it could use the steel smithing perk for improvements and matching set with the other regular pieces. Somebody has reported it as being fixed on its UESP page (https://en.uesp.net/wiki/Skyrim:Ancient_Helmet_of_the_Unburned) but it doesn't appear to be.