@ DaKing's first comment: That's because of the fossils you receive in the originals. Some numbers are carried from a game's predecessors. Again, exceptions are always present.
@ MrKijani: Like I said, there are always going to be exceptions.
@ DaKing's second comment: Dream Radar allows that, and on as early as Route 7 a lady registers the Kami trio to the Pokedex.