This is the way I interpret the info...take it as you will.
When animals are made into familiars (and according to Tome and Blood, you do change an existing animal into a familiar, not summon up a special one then and there) they become magical beasts. I assume the reason for that transformation is to allow for higher Int scores, as well as the myriad special abilities familiars have that normal animals do not.
However, the creature type for Improved Familiars doesn't change, at least in the examples I've seen. My interpretation for that is that elementals, outsiders, dragons, etc...they all already have the capacity in their creature types for high intelligence and spell-like/supernatural abilities. Therefore, a change in creature type is pointless. It adds nothing, and is in fact counterintuitive.
Though, I admit, by this idea the Stirge familiar ought to become a magical beast too. Oversight maybe? *shrug*