A fully charged car battery is 12.8 volts, engine off

When engine is running and you measure battery voltage, the 13 to 14 volts you measure at the battery
is actually the cars alternator voltage. it has to be higher to put charging current back into the
Alarm systems and remote operated door locks, etc. create a parasitic load when engine is off causing a gradual drop in measured battery voltage.
When a car is started and driven only a short distance the alternator charging system does not have time
to fully recharge the battery. When starting, the starter draws high current of several hundred amps until car starts. Cars driven on multiple short trips will not have a fully charged battery. Adding he parasitic loads of alarms, door openers etc when engine is off will result in a very low battery voltage, possibly so low the car wont start. As batteries age their performance deteriorates. Cold weather also affects battery performance.