Has one character learned Wisdom? If so (or has it at a higher level than the other character), they will be getting more experience, so will level up a little sooner.
Do you trade and sell loot with the DK in the lead? That can move stacks to the lead character. I don't mind the gold being moved, but selling a couple alchemy plants or trap material and having the rest of the stack moved from a summoning doll to one of the main characters is a bit of a pain.
