Battery is fine, the system says it’s not.

I have had two devices separately in the last 3 months say that the batteries were low, (a siren and a motion sensor).   Both the Simplysafe base station and the keypad warned me that the batteries were low and I logged into Simplysafe and confirmed the information.  The problem is, none of the batteries in either device were actually dead. When I tested them on a battery tester they were at 80%.  I then added new batteries to the devices and reset them with no problem.

But my real question is -if the batteries are still good, why is the system telling me that they are bad?  Neither of these two devices have ever been triggered since we installed the system, so they have been idle the entire time. Is the battery low warning simply triggered by time and not the actual current battery strength?  In both cases, the system said the devives were ‘offline” due to low batteries.