We still find that differing keyword tools can give you very different results.   Keyword Discovery, for instance, gives (on occasion) millions of searches for a keyword that Overture says nobody has searched on.   Overture also often makes no distinction between the order of the keywords (surely this is precisely what marketers want) so that "UK flowers" will give you exactly the same results as "flowers uk", are they selling these keywords twice on the basis of combined results?

Overture also makes no distinction between singular or plural searches so effectively seems to be selling keywords for each on the basis of the search rate for both combined.

The best policy is probably to only trust keyword frequencies that are universally consistent and don't completely trust the figures from the agency that is selling services based on them.