You would be correct. It's pretty normal to see 12.4 - 14.2 just depending on what the car is doing. That is about what my car does on the LiON battery, as it did on the factory battery.I could very well be wrong, but I think the voltage gauge is displaying the voltage being supplied to the battery. So if it shows a low value, the battery is getting minimal charging front the alternator. When it shows a higher value, the alternator is charging more. That seems to fit what you’re seeing for a battery that’s maintained carefully on a battery tender.
Again, I could be mistaken.