I think some people may get confused by the whole leaded/unleaded thing. From what I hear, you can mix 2 leaded fuels together, or 2 unleaded fuels together and the octane will be an average of the 2. But you can't mix a leaded gas with an unleaded gas and average out the octane numbers, because the octane rating for leaded gasoline is rated differently.
They must use 2 different ratings systems for leaded and unleaded gas, and they can't be directly compared, so you can't blend them and expect the octane rating to be an average of the 2.