The Challenge Rating is only a +1, the Effective Character Level is a +4. If the template is used on a monster or NPC that is swiftly killed or dealt with all the special abilities are blown through fairly quickly. But the immunities, spell-like abilities, possible wings, and other goodies are used by a PC over and over again. Thus they get more use out of them. So the ECL, which is used by PCs is worth far more than the CR, which is applied to monsters.