IIRC, and I may be wrong in this, wounds can only be assigned to models under the template, regardless of how many models the unit has. Any excess wounds are effectively lost. This applies for Str D as much as anything else.
You do not recall correctly. Not only are the wounds not confined to the models under the marker they could indeed result in casualties where none of the models under the template are hurt. For instance you fire a blast at a squad of 10 marines and hit the back 5. You wound with all hits and he fails all his saves still the nearest 5 die with the 5 actually under the template unharmed.
This is because you generate hits on a unit, you roll to wound against a unit and only during wound allocation using the wound pool does anything actually directly effect models. This is the issue with D Weapons. You roll to hit and generate hits ona unit it then tells you to roll a dice for each model hit but no model is ever hit.