It can take up to about 3-hours for a battery to 'settle' after a charge and prior to that, could see something up to the mid 13-volt range, depending on the charger, so anything you read before that point is misleading. An AGM battery is fully charged when it reads in the range of 12.72-12.90vdc after settling. This also depends on the temperature...the lower the temperature, the lower the voltage. That indicated is for 'nominal' room temperature of 68-degrees (the international standard for 'room' temperature). Now, put the vehicle under some load, and that voltage could drop a little (or maybe a lot if the load is high). What would be more interesting would be to monitor the current leaving the battery with an ammeter.
Keep in mind, a high quality DMM might have an accuracy of <1%, but it's not unusual to have it be 2-3x or more (5% on a cheap one is not unheard of) as the price comes down and the time since calibration (if it ever was done well or at all) has occurred.
Then, consider that unless you take the floor up in the trunk, you're measuring in the engine compartment which is probably at least 12' of cable, and a couple of connectors from the actual battery, which can drop the voltage as much as a couple of tenth's of a volt when everything's perfect.