I believe that the lower resting voltage after charge is due to higher internal resistance. As the battery ages, IR goes up and you can see this during the charge cycle where you reach CV much quicker. Since the charger will not exceed 4.2v per cell, and part of that potential is dissipated by the IR, it stands to reason (at least to me!) that the battery will come off the charger at a slightly lower voltage.

My tired 10S packs will be 41.7v after resting off the charger but the lower cycle batteries will maintain 42v.