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Different tools use different google servers to check your rankings. Selecting Google USA as your parameter doesn't guarantee you'll get same results on every tool you use.

(I hope you know Google search results also vary a lot depending on the country you're searching from)

I use Rank Tracker (from SEO spy glass kit). Also, you can add "&gl=us" (without "") at the end of the URL of your google search results. That will show how search results would look in the USA.

Kind Regards,
Zannix

Software rank checker saves a lot of time so I certainly prefer it from searching tens or hundreds of google pages to find my site, additionally it's more accurate since if you are sighed in your google account results are different from the results all other users see because google takes into account your previous search history...