Though many anime derive from manga, not every one of those anime follows its source manga closely. When it comes to anime that are different from the manga, plenty of examples come up. Differences between manga and anime versions can arise for all sorts of reasons: an anime geared towards kids may omit the violence seen in the manga or a studio may need to create filler episodes while awaiting the next manga chapter. You may not realize it, but Pokémon's anime and manga versions differ wildly.
Fans often bristle when their favorite manga aren't replicated faithfully. Imagine waiting for your favorite scene from the manga only to find it's substantially changed or entirely cut from the anime version. On the other hand, such differences mean each version can tell a vastly different story. Just look at the details left out of Akira from manga to anime. Occasionally, the original manga falls short of the anime, which can shift up the plot with exciting and unique filler sagas.