Each and every battery of the same brand and type is different. It is a chemical box. Each cell inside the battery (of cells), is a different voltage even on charge. One could be 2.17 volts. One could be 2.23 volts. All add up to the final voltage across six cells.
On charge which can be a float charge, an equalise charge or a boost charge the battery will be charging at a higher voltage than it's nominal charge which is 12 volts.
Float voltage tends to be 2.18 to 2.25 volts per cell. Equalise 2.4 volts per cell to try and equalise all the individual cells. Boost 2.65 voltss per cells times 6 cells.
A float charge keeps a battery charged enough to restore its amphours and to overcome its internal resistance to stop it discharging when there is no load applied.
Rarely do you boost a car battery nowadays because it is sealed. Boosting or a "gas" charging produces hydrogen gas. and is only done in an open cell "flooded" battery. It is called flooded because the lead plates are flooded in sloshing sulphuric acid.
Once you remove a charge the battery collapses quite quickly to its nominal charge. This could be in minutes depending on the battery. Typically it should sit somewhere above 12 volts. 12.8, 12.6, 12.2.........if it is "open circuit" it will be higher but your car always has something connected to the battery so it will start to discharge as soon as you remove any charging device whether that be a charger or alternator. (Note they call it an alternator but is is actually a DC Generator......). an alternator produces "AC" as do all rotating devices but the AC is "rectified" to DC from a car alternator.....whereas the AC from an alternator connected to a big diesel alternator is left as AC......but then we call it a Diesel Generator even though it produces AC not DC....(very confusing). It's correct name is a Diesel Alternator not a Diesel Generator.........
A battery does most of its work from it's nominal charge to its end voltage. End voltage is the point where your components can no longer work. For example, your end voltage for a starter motor might be 10.8 volts, then the starter motor won't turn any more. That would be 1.8 volts per cell times six. So the battery does all its work between 12 volts and 10,8 volts.
If you are discharging your headlights for example the end voltage could be "zero" because the headlights keep getting dimmer and dimmer over hours until the battery is down to a couple of volts.
As you say it is starting OK.....nothing to worry about.
