Think this way.
A car lead-acid battery is made up of six 2volt cells in series making 12 volts (all wet lead acid cells are 2 volts)
So it will be 12v, or just above, when not charging (6x2v = 12).
Lead acid cells need to be pushed above 2.25V to charge, so 6 x 2.25 = 13.5V
So it must be over 13.5 volts when running or else it won't charge properly.
Engine running volts shows the alternator is doing its job, but engine-off no-load voltage is not a very good indicator of battery condition, you need current/load to see how the volts drop to see if the battery is shagged.