User blog comment:Madman97/The Aedra Vs. The Daedra theory/@comment-3025184-20131010203442/@comment-11345660-20131011033404

Actually, Alduin isn't an aspect of Akatosh at all. He knows Akatosh is depicted as a dragon, and the humans associate him with tremendous power, so he plays off of this and claims himself "Firstborn of Akatosh". He says it himself. Parthunaax will tell you too.