Battery charging voltages will change with temperature, not sure if that alternator you have has that ability though.

https://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

Charge voltage will be higher than the float voltage, 14.8v @70 degrees F. Once you start the car the charging system has to top off the battery quickly so the charging voltage will be high, automotive starting batteries are designed for this. Once it gets close to fully charged it'll drop down to what's called a float voltage, this is the13.8v @70 degrees F or so you see bandied about. As temps rise both of those voltages will drop, as temps get lower both of those voltages will rise. There is no one voltage that should be correct all the time.