I think the difference between anime adaptation of mangas and movie adaptation of books is that movies are visualization of the books so there are so many decisions that directors need to do to adapt a book. However, with mangas, the drawings are already there and the studios just need to expand from there. That is why generally, animes are expansions of mangas (they are more "complete" by filling in the gaps between panels). Sometimes the studio makes too much unnecessary additions that can change the tone and bring it to the other directions, though.
However, there are some exceptions (mostly case by case) for example Death Note tried to cram a lot of chapters in a single episode on the second part so that is why a lot of the scenes are dropped. But for popular battle shonen series, I think generally almost everything in the manga makes it to the anime.