Every CA is individually calibrated before it leaves Grin.
Both CA voltage and current calibration are adjustable by menu so if you have a calibrated source you can easily re-calibrate the CA. However, unless you have a quality meter that is recently calibrated, you can generally assume your meter is uncalibrated and is likely off the mark so adjusting the CA to match may not be a profitable undertaking.
As far as your charger is concerned, recognize that the voltmeter in the charger is at the other end of the charging cable and is reading charger voltage not battery voltage. Beyond any calibration consideration with the charger meter, this means that there will be some small variance ( perhaps .1V or more) from the battery voltage according to the charge current due to the voltage drop across the charger leads. This is really not much, but it will have a visible effect if you are doing voltage comparisons. The longer the leads and the higher the charge current, the larger the drop and more the voltage at the charger will exceed the battery voltage. This difference will change as the charge current tapers off (due to constant voltage charging) as the meter eventually shows the charger voltage approaching the true battery voltage. On the other hand, the CA to battery connection carries negligible current and should more accurately reflect the actual battery voltage.
So - you can finagle the meter calibrations, but you cannot escape the effect of the wiring. This is not an issue in real life simply because the terminal battery voltage is very close to the charger voltage even if the intermediate voltages are a little off. The battery will eventually reach the charger voltage allowing the meter to be 'correct' where it counts.
