When making an opposed test, you compare the actor's statistic with the target's statistic, and add a number of challenge dice to the test based on that comparison. That is, if the target stat is less than one-half the acting stat, add zero dice, between one-half and equal, add one die, and so on. So far, so good.
Basic attacks are opposed tests against the target's defense. So you'd think that this would mean higher defense just adds more challenge dice. And I believe that's so. However, the rulebook section on attacks just says that you should add one misfortune die for every single point of defense.
I strongly suspect that both of these things act in conjunction, but I think it would be great to have a single sentence say "This is in addition to the challenge dice added from comparing your strength or agility to the target's defense." And maybe even "Hitting someone who doesn't want to be hit isn't just difficult: it's dangerous!"