Well it really goes back to the cyclical nature of the show.
My pet theory, to explain why "Earth"-C(ylon) might not be "Earth"-Prime (our earth): Humanity Evolved on Earth-Prime, eventually we colonize other worlds, one of which is Kobol (maybe Kobol is the only one). For whatever reason Earth-Prime falls and its culture falls into the realm of myth and religioni. The Kobolian/Cylon war breaks out, the 12 human tribes head for what would become the colonies, the 13th start head in the general direction that scripture dictates was the location of Earth-Prime. Eventually they find a world that fits the bill-- maybe even an colony abandoned centuries before, they assume it is/call it Earth and settle it. Eventualy they make the same mistakes as the people of Kobol.
Jump to the Colonies a couple thousand years later and once again the pattern is repeating.