Because you'd be able to look at both forms in the dex if you did so. Same species, same dex Number. It only cares that you caught one of the species, not the different forms for the caught/not caught mechanic.
As the one person on gbatemp stated, the 'mons are not stored in the PDR save. They're generated upon importing into one of the Gen V games. The Dream Radar save is TINY. All it likely stores is the species of anything you're going to send.