[DonRotolo]

The short answer is that the influence of the charger - it is supplying a voltage in excess of the battery voltage - complicates getting meaningful data from such a measurement.
Battery chargers generally use current, which eventually drops to near zero, as their monitored parameter to determine state of charge. More specifically, the change of the charging rate is monitored (the derivative (dx/dt) of current) and when it 'flattens' to a specific value the battery has reached a certain charge state.