Batteries are like cats, they have a certain # of lives, kill em dead a few times and the clock is really ticking!
I work aids to navigation, we run batteries 6 years on solar chargers, if the system is properly maintained they are very reliable. This is off the coast of Maine mind you, its god damned unpleasant and dark in January. Heat, short trips, bitter cold, long periods of rest (ie over a week) and rough roads / fast acceleration and deceleration (imagine g forces acting on 50 lbs of lead plates who rely on precise alignment and insulation from each other in a bath of electrolyte to work properly)
All these things conspire to kill your battery. If you drive in conditions like this you will likely see a pathetic battery life.
Now if none of those things apply to you, you will likely see above average longevity.
Parting shot:
Take a battery of a given form factor (size) and chemistry (sealed led acid) and weigh it. if it is heavier it has more lead in it, lead is expensive, only reason to put more expensive heavy **** in a battery is that it makes a better battery, period.