Hmmm From the same town though? one supplier could be using a couple pet stores for their batches. I've noticed I needed to stay away from this town and the bettas because most of them had immune system failure, or couldn't pull through a simple water change.
Dropsy is a sign from an internal problem... Which then can be caused by bacterial infections for instance - so it could be the conditions they were in, how they were kept by the breeder, another fish could have had a sickness that passed to the betta... Hard to know. If you wanted, get your tap water tested, and your aquarium water tested. Ammonia and nitrite can be causing problems too. Sometimes ammonia - or other toxic things are in our tap water that is not good for fish, such as certain metals.