I highly suspect it’s the latter, particularly regions with specialists, since the ‘new’ species need expert identification to differentiate similar species, and document or identify species that have few to zero observations already on iNat.
I’m pretty certain of this based on my experience going through local floras in Monterey, CA and both identifying and observing the species that were missing or had low observations and within a few months got them added to the CV.
I’ve toyed with the idea of picking an under-identified area with a reliable plant list and learning to identify those species with few observations to get them on the map and eventually the CV model.
You’d really need those specialists for verification though, since winging it would do more harm than good to the CV model.