It makes no sense to me that there would be different AI for hirelings of the same class... I mean, why wouldn't you just use your best AI on all of them? But anecdotally, it sure seems that way.
I thought the names on each hireling were just for flavor, but I swear some are smarter or dumber than others. Consistently.
Laerathor (level 7 cleric) is unacceptably slow to react and stupid. He deliberately ignores commands.
Mareth and Arias (level 6 clerics) were more responsive. I mean, not good, but nowhere nearly as bad as Laerathor.
I've summoned all of them multiple times in multiple quests. I don't believe it's anything coincidental or just in my head.