It's is because, in beta gen 1, Pokemon used the "bird" type. Then they say charizard, and that screwed everything up. Sense Charizard isn't a bird, They changed it to Flying-Normal. They kept it that way in later generations too, such as Tailow, and Fletchling. The only Pure flying type, is Tornadus because it doesn't really make sense to have it be flying-normal.