I think it's how well you take care of them and sometimes it's just the luck of the draw. I sold a boat that had well over 15 year old batteries and they since worked fine. (stored them indoors, didn't drain them so often, didn't put them on a heavy load)
I'm trying some MK battery Gel cells on my new boat, the specs look good for them. We been using them at work for several years now. Ever once in a while we get a bad one, but I can't tell you if it was the battery or the person using them.