I didn't remember hearing that there were only five high summoners and when I was considering each FF, I pretty much dismissed that as impossible, because weren't they doing the high summoning for like a 1000 years? Sounds like a plot hole to me. Especially because they act like they're doomed to all die after Yunalesca is killed. Doesn't seem likely if they went centuries without defeating Sin before. Okay, FFX is already pretty filled with holes, so I guess just add another to the list...