[hunter2_]

Expectations of battery life, and typical experiences, assume a certain amount of stress such as extreme temperatures and occasional deep cycling. Someone who keeps their car garaged, never plays the radio for a while after shutting the engine off, and never accidentally drains the battery is going to have an outlier like you're describing. I'm no expert but my understanding is that these types of (potentially avoidable) stresses are majorly significant contributors to a runaway process of internal breakdown/corrosion that determines how much longer it'll work.

Lithium batteries like to be stored at 50-60% charge, while lead acid car batteries like to be stored at 100% charge.