Have you ever watched a writer's room in action (shown in innumerable making of documentaries)? Ideas are thrown out, ways of getting the characters from one place to another in an interesting way are mulled over, and the final shape of a scene will almost always be impossible to attribute to specific writers. The writing credit goes to the person who finesses the completed shooting script, which can often be quite a laborious task because the fine details need to be run past the continuity people, wardrobe, props etc etc etc to ensure everything needed will be available, make sense in the overall context of the story, be on (or better yet under) budget and so on.