The main reason for large extremes is parked and expired domains. Parked domains account for a large chunk of the engine's profits and are classed as searches.
For example, if you visit http://www.kazakov.info, sedo (the domain parking provider) sends a search request to Google to retrieve and display the search results for the term "medical software".
This counts as a search every time the page is visited. This inflates the number of searches for that term and this is often reflected in the Keyword Discovery data (mostly on popular search phrases). This is why you may see searches for keywords such as "online website marketing", for which real people would be very unlikely to search for. This particular term would be likely to appear on an expired domain that previously provided SEO or marketing information.
WordTracker does not collect data from Google or any other search engine that powers the parked domains, so their data does not reflect this source. We have decided to include this data because these results are classed as searches by the engines and if you are advertising on the Google search network, you will be charged for clicks generated by the parked domain pages they serve. This can be substantial because many parking pages can have a very high click through rate.
