You are seeing two different numbers. When the switch is on and engine off, you are reading pure battery voltage available to start the car, and it should be in the 12.5-13.5 range (if fully charged). When you crank the engine, you are reading battery voltage and alternator charging voltage, which should peak out at about 14.5 volts - you have to charge at a voltage higher than the battery is currently holding, or you can't cram any more 'lectrons in there. The alternator has a voltage regulator that makes the alt charge until the battery is crammed plumb full of 'lectrons, then reduces the charging rate to just the engine and accessory load.

In other words, yours is perfectly normal.