Hi Claire
Hope Zak is OK, I think you already have the answer, some wormers do not attack all types of worms and I'm not sure whether some bought of the shelf are good enough, hence why some types are only available from vets.
It doesn't seem to be the case with smalll animal wormers, but with the horses we used to give different types in rotation to make sure all worms were attacked, that was a few years ago and wormers have probably moved on since then.