Why is the waiting period (iddah) for a divorced woman three months, while for a widow it is four months and ten days?

The waiting period (iddah) for a divorced woman differs according to her condition: If she is pregnant, her waiting period is until she gives birth. If she menstruates, her waiting period is three menstrual cycles (quru’). If she does not menstruate, her waiting period is three months. As for a woman observing the waiting period due to the death of her husband, her waiting period is four months and ten days.