Nothing.  The implication is that these locations were used as convenient or plausible addresses for registering fictitious individuals.  It would be more obvious if they used a domicile where someone is already registered or if tens of individuals were registered to a single family home.  You can have hundreds registered at a trailer park, and it's not suspicious, but who's going to verify those are real people.  This is the problem with widespread mail-in voting.  If people have to actually show up, it's much harder to commit fraud.