Oh my God, I asumed that this would be easy to understand.
Ok we have a Conjuror and his pet. Each does 100% damage, but that is only half of the entire dmg encounter. See? 10% of the pet's damage is a 10% reduction. 10% of the Conjurors damage is a 10% reduction. Therefore, easily added to 20%. Just because our pets and us are different, it doesn't mean that we should BOTH get nerfed. However, what you proposed would be nice, I wouldn't mind a 5% to myself and pet. Hope you read this developers. 