This is not an error. All other Flying types have a second type (Besides Arceus, which is basically a Normal-type). For instance, Staraptor is Normal/Flying, while Tornadus is pure Flying.
I'm fairly certain that the reason for this is that using Roost takes away the Flying type, so all Pokemon (except Tornadus and Arceus, which can't learn Roost) were given a part Normal type so they didn't become typeless.
Hope I helped!