The problem is that HD or CR doesn't accurately reflect the usefulness of the critter. For example, the Lantern Archon has 1 HD and CR 2, but is capable of casting Teleport without Error on itself and up to 50 lbs of objects, as well as casting Aid, Detect Evil and Continual Flame at will and being surrounded by a Magic Circle vs. Evil. That's why a Lantern Archon needs Summon Monster IV to work.