Remember, 12.2V for 50% is a resting voltage, i.e. the battery is left to rest, no charge, no load for 24hrs. If there are any loads on the battery, the indicated voltage is lower than resting.
The BMV does not use voltage as SOC, it synchronises to 100% when the charging is complete and then counts energy in and out to keep the SOC.
There 2 reasons for a mismatch.
- The wiring is wrong and some charge or load is not being counted. See Shunt wiring
- The settings are wrong and the synchronisation to 100% is occurring before the batteries are full. See BMV going to 100% early