Mystery about SERP position

[MarketingHelpers]

Could someone please help me out with this one?

One of my Web sites has had hits recently from Google for a particular keyphrase. When I checks my stats, I followed the referring URL back to the Google search and on both occasions I landed on page 2.

I checked my Google site map stats and it says that my site shows up at position 20 for that keyword.

However, I did a search on all the Google datacenters and my site never shows up in the top 25 for that term.

What is going on?

I am not running any Adwords campaigns

Thanks
Bryan