Avg. Item Level vs. Equipped Item Level

Could someone please explain to me how a character's average item level could possibly be lower than his equipped? If the calculation is taking into account the best item level that you possess for each of the 16 slots (with weapon/offhand counting as one but is the average of the two slots, as far as I can tell), then the equipped gear could certainly be lower than the average best, but the average best has to be greater than or equal to any one gear setup.

I'm guessing this is a bug on Blizzard's side, but I haven't seen any mention of it on the boards. If I'm wrong on any of the above, please advise.