Batteries store current NOT voltage. Checking the voltage can give an idea of the state of the charge, but does not measure the ability of the battery to supply current to the starter.
Have a 78 450SEL that I got from the PO because the car wouldn't start after sitting for a couple of days. I checked the voltage and it was OK(12v). Started the car and turned on the lights, voltage was 13.8v. Swapped the battery with one from my other car. Hooked up ammeter to check for current drain, only drawing enough for the clock.
Car sits for a couple of days, dead battery.
Time to put the thinking cap on and start from square one. What is the purpose of the battery? To supply current to the starter. Therefore the battery must not be storing enough current to start the car.
Current flow is affected by the resistance of the charging circuit. Check the cable from the alternator to the (+) side. OK. Check the cable from the (-) side to ground. BINGO! The ground cable is attached to the brace under the battery tray. Brace has rusted to nothingness. VERY high resistance path the ground, therefore low/no current to the battery.
Voltage OK, current not OK.