Under normal circumstances, you start the car & this drains some of the charge from the battery. The altenator goes into high output mode & you will get the highest voltage reading. As the battery comes back up to full charge, the voltage output from the altenator drops back as it now need only to provide output to run the ignition, & any other accessories that you have on.

If you turn on the lights, the aircon, the Hi-F etc. then the altenator will increase the output, including the voltage to keep up with it all.

When you turn the engine off, the voltage in the battery will settle back to whatever your battery will hold. Theoreticly, a battery cell actually provides 2.2 volts. Multiply this by six [cells] & you get 13.2 volts, so your fully charged battery should have pretty close to this.

Glad to hear that the pocket rocket is running again.
Chris