Because they are putting it all in that Scene, Ron finding out about his feelings for Hermione, Hermione telling him about next time to choose her.

At the TLC forum there is this girl who has watched the movie and she said that the only tension in the entire movie between Ron and Hermione was at the Yule Ball, and that's it. She said that it was like in the book.
She didn't notice any other scene were there was tension between those two.