The Aeon Fayth are not actually Zanarkandians, I believe - but people who were born AS (Anno Sin ) and were able to - with the help of Yunalesca, probably - recreate or revive the Summoning as to make them the Third Fayths after Zanarkand and Sin. That is why they do not seem Zanarkandian - they are of Spira, created for Spira and by the people of Spira to protect them and destroy Sin and end the Spiral of Death. Of course, they, too, are immensely old and in need of some sleep, which is why they, in their uncomfortable and tiring Dreaming, decided to take matters in their own hands.

Theory - Jecht & Tidus are not the only Dreamings to have left Zanarkand. The Fayth have been gathering, through all these years, people from the Dreaming to become Final Summonings, which would make Seymour's mommie look more and more like a Fayth - even though a corrupt one.