The PokéDex is written by a kid.
This can be seen by this quote from Professor Oak: "To make a complete guide on all the Pokémon in the world… That was my dream! But, I'm too old! I can't do it! So, I want you two to fulfill my dream for me! Get moving, you two! This is a great undertaking in Pokémon history!"
It implies that Professor Oak didn't fill the PokéDex completely, but wants you to fill it. Usually the protagonist is around 10 years old, which is the reason why some of these PokéDexes are fake.
Say for example Banette's PokéDex: It's literally a ghost story some other person probably passed on. It's safe to say that most of these are fake. Though since Pokémon does have the Ghost type and is fantasy, so it could make sense. Which is why I want to show another example, which is about how PokéDexes are over exaggerations, like how Magcargo's body temperature can reach over 18,000 °F. Or they just don't have any proof at all, and are merely hypotheses.