I have used Wordtracker, but after testing it on some less popular keywords it doesn’t appear all that accurate to me either. (haven’t bothered yet with the highly competitive ones) Another thing that brings this into question is that there are certain typos that show up with more searches than the original phrase. That shouldn’t happen.
Programs like Wordtracker are good for giving you suggestions/ideas but you need to apply some common sense to the results, as in what word(s)/phrases would you personally use if searching for this topic. The stat program I use has helped in that is gives the search term that the visitor used in the respective search engine to find my site. Interesting sometimes what people type in.