Here's the logic behind it. The Ravage Agent does trigger even when a model just activates in it, but the Ravage Agent Spread happens at the beginning of the next turn. So you can lose even more than one model to the Ravage Agent, but you are able to reposition your troops, so they are not caught again by the next spread. The board in this scenario can become externally deadly if players are inattentive about unit placement. And that's the point. At the same time, that is why the Ravage Agent deals a hit not an automatic wound. A 6 does not create a casualty, so the Ravage Agent Spread is not triggered. Also, all Regeneration rules etc still apply. An additional fail safe I thought about was if you roll a double, it means that only one marker gets only a single additional inch... but for the sake of the theme of impending doom I decided to roll with it.