[RWT]

GM has a "Smart" charging system that uses up to 9 different modes (depending on vehicle). It uses these system modes to maintain a proper state of charge (SOC). If you are the least bit old school it's really strange.

Flooded batteries don't handle discharge (deeper cycling) well so older charging systems generally ran high a lot of the time. AGM batteries, however, are very sensitive to over-charging but handle deeper cycling better. AGM batteries also charge much faster (less internal resistance).

The smart charging system will charge high (14+) after starting (starting mode, one of the 9 modes) but it won't stay there long at all because the AGM battery charges faster and doesn't like being over-charged.

When "cruising" along the charging system will go into "economy mode" to save fuel (alternators use horsepower) and your voltage output will drop lower. There's even a mode where it doesn't charge at all. You may even see an increase in output when decelerating as the system uses the RPMs to increase your charging rate while not impacting fuel usage (kind of like regenerative braking, if you will).

So watch your readout for a high number (14+ probably) upon start but diminish relatively quickly. While cruising you likely see relatively low charging rates. The smart charging systems tries to maintain a steady "state of charge" (SOC) by reducing the time spent at high charging and discharging rates.

I am not saying your battery is not going bad because sometimes batteries just do so. However, it's likely the reading is in the 12s because your battery is fully charged at the time and the system has gone into economy mode as you cruise along.

JMHO
No expertise implied or expressed