The Ron Weasley in the films is just not the same character as the one from the books, that's all there is to it. Whereas Ron in the books is loyal, strategic, and knowledgeable about the wizarding world, movie Ron is a bumbling idiot who has a really inconsistent character that varies wildly from film to film. None of this is Rupert Grint's fault, to be fair - it has a lot more to do with screenwriter Steve Kloves, and the fact that he favored Hermione. There's a very delicate balance in the main trio, with what each character brings to the table. Where Harry has stone cold courage and intuition, and Hermione has book knowledge and pragmatism, Ron is the chess player, the tactician, and the one who has a different frame of reference having grown up in the wizarding world . But when you make Hermione good at everything, Ron's significance to the group suffers. In the films, you start to wonder why they even need him around. But the real difference between his two portrayals can be seen in an exchange in Potions class, when Hermione answers a question and Snape calls her an insufferable know-it-all. Where book Ron furiously snaps back at him, "You asked a question and she knows the answer! Why ask if you don't want to be told?", film Ron turns to Harry and says, "He's right, you know." That tells you all you need to know about how the character was changed, and not for the better.