The state of charge % as shown on most battery chargers, is entirely based on initial voltage when the battery was connected and the charger plugged in. There is no electronic magic going on where the charger delves into the battery and can see its state of charge or state of health, the voltage gives a general idea of the state of charge, but since full charge resting voltage can vary from 13.16 to 12.6 depending on the battery the whole % is relatively asinine, especially when the user takes it as verbatim.
IF the 6v/12v combo charger thought it was connected to a 6v battery and saw 6.00 volts it would NOT say 98% charged, as a 6v battery is fully charged in the 6.4v range. If it was a combo 6v or 12v charger and thought it was connected to a 6v battery measuring 6.00v it would say ~50% charged.
Use a real voltmeter on the battery terminals.
Do not trust the % screen on chargers, or the voltage for that matter, especially if it is a Schumacher. My Schumacher was so hideously wrong, even when the display still worked. The display no longer does, but the charger still works, I just use a separate voltmeter and ammeter to see what it is doing, mostly if I clicked the buttons enough times to choose the 2 or 12 or 25 amp setting as those buttons lights no longer function anymore either.