Most auto/marine batteries are made by three manufacturers, Delphi, Exide, and Johnson Controls. Each makes batteries sold under different brand names.
Delphi makes ACDelco and some EverStart (Wal-Mart).
Exide makes Champion, Exide, Napa, and some EverStart.
Johnson Controls makes Diehard (Sears), Duralast (AutoZone), Interstate, Kirkland (Costco), Motorcraft (Ford), and some EverStart.
As you can see, Interstate and Duralast are made by the same company, Interstate just costs more than the Duralast.