Do not confuse battery voltage and charger voltage. You can have the charger voltage at 14.8 and the battery voltage at 12.1 and now you have a good "spread" (voltage difference) so amps will flow. You need enough spread to allow at least as many amps to flow as the charger is limited to (being "current limited " by design as indicated by its amps rating)
So you get constant 75 amps till the battery voltage rises near the charger's voltage. Once the spread in voltages gets too narrow to allow the 75 amps, amps will taper. As battery voltage rises with rising SOC you see fewer and fewer amps as the spread narrows.
See my ugly graphs for what that looks like.
OK now for the tricky part. When actually charging you have the wires from charger to battery and the voltage should be the same at each end of the wires, but they are not the same. You have voltage drop which is more with higher current.
So you cannot measure charger EDIT voltage vs battery voltage END EDIT with a meter while all this is going on. The meter readings you get are a sort of average between the two as they try to equalize their difference, with also the voltage drop difference playing its part.
As SOC rises the voltage spread narrows but so does the voltage drop because amps are tapering so eventually when the batts are full (accepting no more amps) the two voltages are the same at each end of the wires and that would be the charger's voltage if it is still on.
