I'll repost this topic here, because it doesn't get as much attention as I'd hoped it would.

It is my understanding (according to this: http://elderscrolls.wikia.com/wiki/Soul_Cairn_(Dawnguard)) that the Soul Cairn only takes the souls of a used soul (meaning: soul used for enchanting a weapon or armor). The souls that are not yet used, remain in the soul gem, until it is broken (the soul is then free) or is used for enchanting (the remains of the soul go to the Soul Cairn for an eternity).

Thoughts on this?

"The nature of the soul is not knowable"

I think there's lore to support that souls can remain after enchantment. Perhaps it's soul gems that are used to recharge items that release souls into the Soul Cairn?

The lore on this is all very vague. I wish this subject had more lore.

You'll find that Bethesda will be pretty careful about leaving their options open and not boxing themselves into any lore prematurely. And it leaves things for players to speculate over.