I like to think of it this way:
The Ranger has the highest damage potential, but also the highest risk for failing to do any damage since nearly all of the Ranger's attacks are made against Armor Class, which is most often the highest defense a creature has.
The Warlock has the most consistent damage potential, since the warlock can pick and choose powers that affect any of the target's defenses. If an enemy has a high armor class, they can attack their reflex or will defense instead.
Your overall damage isn't only a matter of how much damage your abilities do. Damage is a factor of the potential damage of your abilities and the chance for those abilities to hit. The Ranger has more potential damage, but the Warlock has a higher chance for their attacks to hit.