In the Fellowship of the Ring movie, it explains how Bilbo got the ring:
Its time had now come. It abandoned Gollum. But then something happened that the Ring did not intend. It was picked up by the most unlikely creature imaginable. A hobbit, Bilbo Baggins, of the Shire.
In The Hobbit: An Unexpected Journey movie, Bilbo and a goblin falls after they fight. And Bilbo woke up under the mushrooms. Goblin was unconscious. And Bilbo sees Gollum. It was try to drag it's own place for eat. Goblin was struggled and Gollum starts to hit it until it dies. In this figth, the one ring falls from Gollum's pocket. Gollum starts to drag Goblin again. Bilbo waits until he left, he stands up, pick his sword under the mushrooms and then see the ring. He take it and put in his pocket.
The Book: Bilbo awakens in total darkness; it's so dark he literally cannot tell the difference between having his eyes open or closed. He gropes around on all fours, and his hand accidentally brushes against a small ring of cold metal. He puts it in his pocket without thinking and continues on his way. He sits down for a moment. He feels for his pipe, then for his tobacco, but cannot find a match. That's the last straw for Bilbo Baggins' hopes, and he despairs for a moment. He draws his sword, and noticing that it "shone pale and dim before his eyes," found enough hope to continue. Using his sword as a source of light, he trots along.
Also there is a video from youtube which explain my question.
In my opinion, these 3 stories are quite different. I don't expect they all should be exact the same but I didn't expect they will be quite different.
Is there any reason makes this situation different each other in films and movies?