I've used the SEO Book Keyword Suggestion Tool and the Free Keyword Suggestion Tool on the WordTracker site, but a lot of times, the numbers they report don't agree with what Google has.

I would see like 2,000 searches in WordTracker and then go over to Google traffic estimator and see 'not enough data to give estimates'

So what's going on? I'm inclined to believe that Google is correct and WordTracker is wrong, but why is it wrong? How can it be a popular tool if it's wrong so often? Is there any other service that I can use to cross check the numbers for confirmation?

Thanks.