One possible scenario - and this is totally hypothetical ! - out of the millions or billions of possibilities would be something like the code had a bug that caused the system that read the battery voltage to not recalibrate itself as intended. So if the ADC that converts the battery voltage to a number for the BMS changed over time (and virtually all silicon devices change characteristics over time, temperature, use, etc) then the voltage reported to the BMS would be incorrect. Now imagine that there are multiple vendors and batches of these ADC parts. Then different cars would exhibit different behaviors over time. Fixing the software to recalibrate the part as needed would eliminate the problem and the BMS would get a more accurate reading of the actual battery voltage, as it would now be unaffected by the errors in the ADC.
Seriously doubt that system designers would not have the considered the potential of an ADC's reference voltage drift over time. Most likely
there's redundancy in the system to monitor that and correct for it. The key point, though, is that DAC functions today are integrated in the
microcontroller (processor) chips, e.g. used in the BMS and other vehicle ECUs. Furthermore, the supply voltages of processor chips are
very tightly regulated. Given the CAN bus system of present day vehicles, various ECUs can cross reference battery supply voltages,
e.g. VCM versus the BMS voltages as the ECUs communicate.
Most likely the BMS of the 30 kWh Leafs had the same firmware as the earlier Leafs, with just modifications to key parameters
for future battery upgrades being stored in flash RAM and easily updated via Nissan's Consult diagnostic tool. So it's highly doubtful
that only the 30 kWh Leaf's BMS had a firmware design problem that required an update. It's more likely an issue with the 30 kWh
battery and not an initial firmware design error nor an electronics problem. It would have been easy for Nissan to update the BMS
firmware to test for certain battery conditions/parameters that may only occur to some of the 30 kWh vehicles, and then only tweak
the necessary BMS parameters. If those vehicles never entered the modes tested for, the BMS firmware tweak (patch) wouldn't run.
#1 Leaf SL MY 9/13: 76K miles, 47 Ahrs, 5.0 miles/kWh (average), Hx=70, SOH=78, L2 - 100% > 1000, temp < 95F, (DOD) > 20 Ahrs
#2 Leaf SL MY 12/18: 10.3K miles, SOH 109Ahrs/115Ahrs, 5.2 miles/kWh (average), DOD > 20%, temp < 105F