Ohms law v/i=r in the case of a battery how much current is in the battery dictates how much voltage it can produce, and how much resistance (current drawn) will have an effect on that voltage too.
A fully charged battery would normally be 13.5vdc 12v could mean that the battery is discharged and requires charging. It could also mean that the battery is no good ( There is a reason they are supplied partially charged). The battery is supplied to the distributor with a charge but not fully charged. While its sitting on the shelf it will over time discharge, but it takes a while and its not good for the battery. We as a company have a high turn over of batteries, so we never have a discharged battery on any of our van stock, or in our stores they dont sit around long enough to.
The manufacturers recommended life span of a battery is 4 years, but because a battery is a consumable it entirely depends on how much it has been used as to how quick it becomes past its best. I am not talking about how many power cuts either, mains is never a constant and you get spikes and dips, so how clean the mains electricity is that is supplied to your alarm effects the life and condition of battery.