No, the commercial charger likely wasn’t off by much, if at all. Charging hardware itself usually has an accuracy within three percent, in terms of energy transferred at the charging port.
The real reason for the discrepancy is that you lose some energy to heat in the onboard charger. According to Kia, for instance, it’s typical for the onboard charger to lose 14 percent or more of the energy inputted on the way to the battery pack’s cells. Factor in charger inaccuracies (Kia notes that 3 to 5 percent isn’t unusual) and you could end up ‘officially’ putting in well over 15 percent more energy than the battery’s capacity, to restore it to 100 percent.
