I've been thinking about it for some time now and came to the conclusion that chin ups and pull ups are ego-lifting partial repetitions, even when the chin is over the bar or when the chest is over the bar it's not a full repetition because the back muscles are contracted at less than half of their full range of motion.
An actual fully contracted pull up would be with a bar under the belly button and elbows bent completely behind the back like in a dumbbell row. But people just do simple chin over the bar pull ups and get a good back workout out of it. Why doesn't this apply to other exercises, if pull ups build the back muscle with just some partial reps so why don't partial deadlifts or partial squats build muscle too?