SO users are from around the world. That said, different regions have different idioms and expressions. This means that various SO users may phrase questions differently, causing the "Related Questions" box to be inaccurate or miss related questions. This also adversely affects the "Related" column in the sidebar.
Consider this a request that something be done, although I'm not sure exactly what, though. I was thinking of a "regional" or "local" lexicon setting which would be tied to each account, or perhaps a master list of interchangeable words so that the "Related" views are more accurate.