Sounds pretty opaque.
Fortunately for me, the booking window for the flights I want is still a couple weeks out, so I have time to try to clarify things a bit. I guess the best way to know would be a simple test: on a day when I see availability for both the cheap award seats and the cheap paid seats, I can try buying the two cheap seats, and then immediately check to see if the low-level award seats disappear (I assume the inventory management updates in real time for that). If the low level award seats don't disappear, then I can conclude that they're coming from two separate buckets (or for all intents and purposes, that's how it'll work for me). If the cheap award seats vanish as soon as I buy the cheap purchased seats, then I'm going to need to come up with a Plan B.
As long as I don't pick "Saver fares" for the purchased seats (I never would anyway), I figure they're fully refundable within 24 hours and it's safe to do the test. Ah, experimentation...
I will report back with the results in case anyone's curious.