Talos (Tiber Septim) is not from Atmora, he is from the Alcaire of High Rock, with the name Hjalti Early-Beard, and he was either Nord or a Breton. (Note: Tiber Septim's life is very obscured and contradicting so I won't go into it more. About who can be Dragonborn is not something you get from you ancestors, the gift of having a Dragon Soul is given by Akatosh, it is not something that is limited to a specific race. And just because the Default race is Nord have nothing to do with it, the race is most likely set based on the race that is 'native' for the province. And it some seem to mistake the dragonborn rulers with the Dragonborn, the dragonborn rulers of the Septim have the blessing of Akatosh (because of the gift given to Saint Alessia) the Amulet of Kings, the blood of Akatosh, and therefore they have been named the dragonborn. The Dragonborn you play in Skyrim have the SOUL of a Dragon, and is a whole other thing, think dragon in human/beast body.