Assuming equal skill (and damage profile of the class - pull cadence hurts some classes more than others), around 15 ilvls would be about 15% damage difference. Given that dps differences we see often are far greater than that indicating skill is a more dominant factor, it is perfectly possible for someone who is playing their class well with 15 ilvls difference to outdps a poorly played higher ilvl players. I regularly outdps players in the lower key range on my undergeared alts.