While they don't have to keep the Cap and FO staffing in perfect balance, they don't build Cap only and FO only pairings in non-augmented flying. So, they will need the staffing levels between the two seats at each domicile to be close enough to allow the pairings built in PBS to be covered. So after you account for the increased Vacation available for the more senior Captain seat, you will see that the staffing levels across each of the BES's for 737s and A320s has balanced staffing for Caps and FOs