People from the Source are more 'complete' than everyone from the First, but evidently that means sod-all, because there are... several people on the First, actually, that give us a run for our money.

Personally, my theory is that relative soul density means little to nothing (at least, in ways that are relevant to most actors in the game's story), but is a metric the Ascians can latch onto as 'proof' of their inherent superiority. So 'more soul equals better than' is not actually a rule of any kind (in fact, we have a LOT more exceptions than we have adherents to it), but because it sounds right and is something the Ascians can claim about themselves compared to others, it becomes the be-all-end-all.