I think you are right about that. However, I was testing for changes in the results of the combat and not changes in the duration of combat. I strikes me that whilst increasing the defence stats of a unit would prolong the battle, the side with the higher defence should still win (i.e. kill more enemy) assuming everything else is equal. So increased defence should should endow a discernable combat advantage, and +6 should be a fairly large advantage when using peasants. I suppose I could redo a few of the tests...