How is it possible for a Dunmer to be Dragonborn? They don't believe in Akatosh and the dragon blood is a "gift" bestowed to certain mortals favored by Akatosh. Why would Akatosh give the dragon blood to someone that doesn't accept it's existance?

They believe in Akatosh, orthodox Velothism or whatever you may call it just says that he's not fit for worship.. or something.

Becuase that person NEEDS the power.