When I researched the assignment history of a patent I found one assignment (from assignor A to assignee B) and a later assignment (from assignor B and C to assignee D).
Where does C come from?
The simple answer is that the USPTO Assignment Record Database is a complete mess and cannot really be relied upon to point you to the correct owner. There are many "off record" transactions. In your case, the assignment from "C" may also be something like a quitclaim deed.