It can happen so quickly... You have a Magic-User NPC, something comes up that complicates the story and you think to yourself "Oh bollocks, this NPC is a spellcaster, let's just have him cast a spell that fixes the problem." (Like in my case when a certain Wizard NPC was able to teleport a player's new character into an isolated place where the other players were!)
And you move on with your story and everything is dandy. The problem has been fixed, hasn't it? But you have just overlooked the one you just created, if you care about story consistency at all! And the more you use the Wizard NPC as a 'Get Out of Jail for Free' card, the worse it becomes!
See, after a while you'll need to give actual stats to that Wizard NPC. Maybe not today, maybe not tomorrow, but at some point in your campaign, if it runs long enough, it will be relevant to do so. And it's at that moment when you realize that for that Magic-User to have done all that you had him do, he'd have to be some sort of Gandalf, Merlin or Elminster!
It might not be a problem in some games, but I know that in mine it has become a bit of a problem as I prepare future events. (Especially since in this case we're talking about a married COUPLE of Magic-Users, so there are TWO of them!)
Sure, I could go the way of 'Lamentations of the Flame Princess' and just say that the magic of NPCs doesn't need to be explained the same way as that of PCs, but in this case it doesn't feel like the appropriate solution to me.
So anyway, for a proper solution let's not look any further than Rudolph Van Richten, Ravenloft's most popular hero!
So what level is this veteran monster hunter, the paragon that all heroes of Ravenloft aspire to emulate? Well, according to 2nd Edition... He's a level 3 Thief. Huh??? Oh, they say it's because he gets Level Drained often by fighting the undead. O.k then! Just have the NPC who is way too high level mention next time that he came across this guy:
There you go, problem fixed! Now your Magic-User NPC goes from this...