These are great thoughts, thanks guys!
First on experience. I think that pawsplay is right, an NPC living an interesting life can certainly gain levels over time. I also think Wolfwood is right, and NPCs probably have some sort of method of gaining experience from training (PCs would never do that, of course!

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But, the reason I beg this question is because Monte Cook says that in Ptolus, the conceits of the game are the conceits of the world. Certainly, if the guard are asking for volunteers to be polymorphed into trolls, I think the NPCs have some perception of a vocation to pursue. Certainly, a Rogue could make a conscious decision to take levels in Wizard. But why commoner? BTW, the NPC is listed as Rogue 2 / Commoner 6, leading one to believe the rogue levels came first.
Most NPCs, while not being adventurers, have to make decisions about "their character" in any case -- where to put skill ranks (obviously in profession-related skills), what alignment to be, how to spend their meager earnings. Why is it hard to wonder how they make their decisions on which classes to take when they level up?
I guess the thing that bugs me is there is no redeeming value to the Commoner or Warrior class, based on what else is available without prerequisites. I can see someone taking Wizard / Expert, or Rogue / Adept, but not Rogue / Commoner ...