Ah, that makes more sense, especially if understood within the context of maintaining a population of susceptible worms 'in refugia' on the pasture already.
I still don't understand the OP's vet's recommendation to use Zolvix as a first resort, and on low worm count though. Wouldn't it make more sense to either not worm at all, or to use one of the older products, and then re-sample to check effectiveness?