Don't ask me why, but I had a 1000 in my farm tractor which I only use once in a while. It can sit for as long as 6 months sometimes (if it doesn't snow during the winter time) and the 'ol thing lasted 11 1/2 years (documented). I replaced it with about an 850 (the parts store quit keeping 1000's) a few years ago and the same thing. Even at a 6 month stand, so far so good, it always starts like I just ran it the day before.
That's my farm tractor, now my truck, lol. I bought that battery new in February of '09 (also documented). It beat my tractor battery and is still going at 11 years and 9 months. Same thing, sometimes sits for as long as 3 months and starts just fine.
I believe with Batteries it is just good luck. Some are evidently made better and last longer, others don't. As much as I like Delco, I have been hearing some negative reports on them.
When buying one for my tractor or yard wrecker though, I always go with the highest CCA's and it seems to pay off.