Last week we lost few ranks in Google (when checked via Google USA in Google Global tool), but when searched in google.com from Europe or India, the ranks remained the same. Is it possible this was due to Penguin 2.1.?
There are quite a few factors that go into calculating search rankings--which aren't limited to search engines tweaking their algorithms. All you can do is practice good SEO and post good content.
Remember--other sites are probably employing various SEO techniques and also posting good content, so sites rise and fall all the time. The web is in a constant state of flux.
Sorry that this doesn't really answer your question directly, but there's no good way to definitively determine why SERPs change.
Some (few) of my ranks completely disappeared from the US Google (checked by Google Global plugin), but not from any other google.com. My question is basically — is it possible that this is due to Penguin 2.1., or did the Penguin affect all local googles at the same time. I can't say exactly when ranks disappeared, but it was recently. I'm not worried about my content. Also, in three months we did maybe 150 links from established and related sites spread to all subpages of our site and we used different anchor texts.
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