Clearly there is a role for both genetics and the soul. Transmigration skews this because those who have been been resurrected all have the exact same strength (except Lanfear who had special circumstances). But for those born naturally there is obviously an element of genetics involved in their strength. I'm fairly certain there is an RJ quote about this out there as well.

We hear about how the "old blood runs strong" in the TR and we've seen the Sea Folk women have entire family lines of very strong women.

1000 women of a potential field of perhaps millions is a tiny sample, especially when regionally skewed. All that added to the facts the author laid out make it obvious to me that these Novices are non-representative. Personally I think most of the purpose they served was to show that there are a huge number of potential Channelers in the world and that women like Sharina could be hidden almost anywhere