Bro VR2 also commented the same that maybe my car has some minor electrical leak. At least it can still last over 2 years on mine though.To a certain extend it sounds like your car may be quietly leaking some current. Not exactly Marathoner's fault. But like you my Marathoner experience wasn't good, battery didn't last a year even. And because of emergency didn't have time to claim it.
But its the cheapest battery so far, I'll be using it again on the sampan...
Century Marathoner is not the cheapest, got other brands cheaper.