Why does the waiting period (iddah) for a widow differ in duration from that of a divorced woman?

The waiting period (iddah) for a woman whose husband has died is four months and ten days, unless she is pregnant; if she is pregnant, her 'iddah ends upon delivery. As for the divorced woman, her 'iddah is three menstrual cycles (quru'), unless she is pregnant, in which case her 'iddah ends upon delivery. If she is one who does not menstruate, then her 'iddah is three months. The wisdom behind the difference in the 'iddah periods is a submission to the command of Allah.