Interesting. So you're saying that according to Google Adwords Keyword tool, that the keyword for one of the pages on your site is listed as having lots of traffic/competition. But even though you rank highly for that keyword, you are not seeing the traffic?
I don't think the Adwords Keyword tool is trying to defraud you. However, the explanation I come up with are a possible regional search result skew. In other words, if the traffic is coming from the UK, but Adwords is measuring US, there may not be as much traffic.
Also, the Adwords tool is not intended for publishers. It's intended for advertisers to measure campaign cost vs effectiveness.
The moral to the story is to keep writing lots of high quality content. Even if you don't win it big with one keyword, the more content you have, the chances of winning some place else increase.