Consider the below graph of LEAD ACID charging efficiency:
Any individual battery cell in a series battery bank that is lagging the others in SOC will be charged at a higher efficiency rate. This process would tend to keep all individual cells in the same SOC range. It would also tend to keep them from deviating in the first place. This would also apply to parallel battery banks.
This process is more pronounced if the battery bank/banks are routinely kept in the 75-95% SOC range.