One of my student's work is making Japanese subtitles for English movies and television shows. During the course of my lessons with her, I've come to learn some of the reasons why subtitles in Japanese don't seem to match the English that is uttered. Incidentally, it's not only English speakers who notice the discrepancy between subtitled content and spoken content. One of my other students asked me just this week why what sounds like several paragraphs of English are uttered and one Japanese word appears on screen.
According to my student who does subtitling professionally, the biggest reason why these discrepancies occur is that there are strict rules about subtitling and the number of characters that can appear on the screen within a certain span of time. My student said that this rule was created in an attempt to ensure that everyone can read what is written in each subtitle before it vanishes from the screen and moves on to the next one.
Another reason is that certain ideas simply do not come across the same in Japanese and it's impossible to translate them in a meaningful way without mutating the intent. I've also discovered based on some of the work my student has done that the person doing the subtitling sometimes just makes a judgment call about what they should make the effort to translate and what they should simply sum up as succinctly as possible and move on. This is a decision that's made in part because of cultural differences and in part because of the character-limit rules.
Sometimes, I read the subtitles on television shows or movies and have come across some discrepancies that really don't seem to fit into these categories. One of the earliest was a movie which mentioned a tuna sandwich which was written in Japanese as a ham sandwich. I also ran across a few recently where a chocolate fountain was described as a chocolate cake. More oddly though was a character talking about people asking him about the war in Afghanistan being translated into something to do with Harry Potter.
When I asked my student about these sorts of changes, she was stumped. Most of the points I catch on the odd occasions when I read what I can of Japanese subtitles are unimportant details. It doesn't matter to the story if these small discrepancies exist but, on the other hand, it wouldn't seem all that much of a burden to just translate the dialog "as is."
Of course, anyone who has been here for awhile knows that it's not only subtitles that are altered but also movie titles. Back when I was working at Nova (in my first two years in Japan), a student asked me what movie I'd watched recently and I said I'd seen "Awakenings" with Robin Williams and Robert DeNiro because I'd read the book it was based on and found it interesting. The student had no idea what movie I was referring to because the movie in Japanese is "Leonard No Asa" which is like "Leonard's morning". Even when a student can translate the English into Japanese, the title at times does not match. There's an interesting list of English titles and their Japanese equivalents here on another fellow's blog.