Why is it that mage characters in Skyrim, such as Pyromancers, Cryomancers, and Necromancers demonstrate such prowess with Destruction magic in their living state. Yet when they are slain, and I restore them to life as a Thrall, they seem to only use far weaker spells that they initially utilized. Like today, I was doing the quest "Dampened Spirits", and whilst I was in the process of eliminating Hamelyn, he was continually sending Adept-level Fireballs my way. After which I brought him back from undeath to fight for me, he now just uses Apprentice-level Ice spikes, and nothing more than that. This also applies when I thrall high level mages. In life, they would show proficiency in adept level destruction spells, and in death, they seem to lose most of their talent and can only cast weaker spells.
This problem seems to only apply to magic users though, as undead warriors still wield the same weapons they did when they were among the living. So does reanimation make these mages dumber as a result? Do they lose a large portion of their magicka reserves?