First off, the whole 'Talos of Atmora' thing is false -- or at least, Hjalti Early-Beard of Alcaire was canon. So Tiber Septim/Hjalti was from High Rock, and more than likely a Breton. Second, Cyrodiil has always been the seat of the Empire. It's in the center of Tamriel, and basically everything Saint_Jiub said.
I'm not convinced he's Breton, just for being from Alcaire. The ghost of Hroldan seemed to have known Hjalti from Alcaire and trained with him, and he's a Nord too. Hjalti is a Nordic name, and even the Talos statues are a sort of a Preset 1 Nord, with a beard.