Board Thread:General Discussion/@comment-24.157.36.219-20130614155339/@comment-6881629-20130614180742

StormKhajiit wrote: Tiber Septim, founder of the Septim Dynasty of Tamriel, was the next... Also from that page...

Copy/paste from the wiki page -

"The Dragonborn is a rare individual who has been born with the blood and soul of a Dragon."

"Dragonborn scarcely exist, and it is rare for more than one to appear within an era. They can belong to any race or gender, since the Dragon blood is a "gift" bestowed to certain mortals favored by Akatosh."

If all the Septims were Dragonborn then it really wouldn't be that rare.