Inconsistent Item Drop Rules?

Is there a reason that, when you only use one character (without a hireling), you get a pretty even split of different class drops, but when you use a hireling, almost all of the loot matches the two characters you are using?

To my knowledge it is set up as follows:

Single toon: Random class drops
Two different class toons: 50% chance of either class
Two of the same class toons: 100% chance of drops being that class.

Correct me if I’m wrong.

It appears to be something like that, excluding drops from feats. Note that your third rule is really just a variation of the second rule. It just seems like a strange change.

Correct. However it’s important to note that the big difference comes from what you intend to use the hireling for. If you are farming a specific item or class type you would want to have a hireling of the same class so as to reduce the size of the potential item pool. If your grinding levels/floors than it doesn’t really matter.