It is a bit of a odd choice and its hard to tell if they had a reason for the choice just wanted to spread the tempering out or even if they just clicked the wrong button. I explained it away in my head that because the technique was so old that there was some sort of mystical element to it, espcially with what you have to do to make the Nord Hero Weapons.
Although it was from the time of the Dragon War theres no scale or bone used in it though, if i was going change it i'd put it Advanced Armors (as it's a mix of leather, iron and steel).