I have a slight hearing problem, so I usually watch TV and DVDs with the closed captioning on. For the most part, the subtitles are pretty true to the spoken dialog. I think, like Rewboss said, that it's a matter of accents and speech patterns. Some people drop the last letter on words, some slur syllables together, and some talk very fast. I'm a native speaker, and there are some films that I've seen at the cinema where there were entire conversations I didn't understand until I later watched at home on DVD the subtitles.
