When testing battery voltage, it need a load for short time before testing with volt meter, otherwise it will read 12 volts or higher. You be surprise how just a couple year old battery end up at 12 or less. Each time, you let your battery run down to 50 percent, ( AGM 30 percent) you are slowly shortening the life of the battery (Sulphating).
Regular charging will not fix it, so it never charges to 100 percent, despite your charger think so. You would need a conditioner charger to fix it, (NOCO) assuming it not damaged too much. Perhaps the Repair option my salvage it?
A handy tool to test the life of your battery, is one of these battery analyzer tester. I used this one, great for RV batteries, I used AGM so try not to go below 50 percent before I charge up with my Honda generator, though I am good to 30 percent for AGM:
Ancel BST200 Car Battery Load Tester and diagnostics.
I have Diesel Truck, winter can get very cold and despite two batteries 80 x 2 amp hour, it is common for my battery to drop 12 volts or lower. When starting up, it heats the air intake for combustion to occur when cold, causing a load to drop to 10.8 volts, then gradually climbs to 11, 12 to 14v (This is normal), so during the night, my park mode will shut down. Especially if it happen often and doing short trips, I would charge up the battery with my NOCO G2600 battery Conditioner.
Took 8 hours recently to bring it up 100 percent, despite being 26 amp charger, it goes through several stages of optimizing. There is a stage where it uses low voltage and low amperage to bring it back to life. It does 9 stages to finish. Regular chargers don't do that. Alternators/Generators don't do that. They are good for just maintaining, not a charger. Common misunderstanding.
Regular flood/Lead battery discharge quickly and quicker still when very cold. If it sat there for 6 months or more, your fully charge battery would be below 50 percent and getting damaged. AGM slowly discharge and handle cold weather and can be stored for a year and still be mostly charged. AGM suppose to be several times quicker to charge, but I haven't seen that, not with conditioner chargers.