Monsters in the wild also gain no additional defense or attack as they level up. You are looking at level milestones in the XML which determines the stats of a creature when it spawns.

But this is a bit curious. As a Procipinee fan I noticed something regarding this subject. Procipinee has a +2 to summoned units level, and when she summons a shadow warg she summons a Level 3 difficulty with Level 3 experience shadow warg and not a Level 1 difficulty shadow warg. Even more, if she learns the summoning ability then you can banish the summoned shadow warg and summon a Level 4 dofficulty with Level 4 experience shadow warg.