You have to understand Attack and defence rolls (both sides) are simultaneous. For ease of play, the attacker rolls first. His hits are counted. The defender chooses his losses. But they are not dead yet. They still get to shoot back. If he puts the units he chooses to lose on the Battle Strip, they can be removed ATTER he has rolled his defence.
Some units have different Attack and Defence rolls. This difference simply reflects some units being better suited to Attack or defence.
After both sides have rolled once for each unit and the losses are removed, the attacker decides if he wishes to continue for a second round. Of course if one side has lost all his units, you already have a winner. If it was a land battle and the territory was captured from the defender, add the income to the Tracker.