From time to time I'm seeing the following behavior: A result which should appear on the top ten is filtered out. It's neither on the first page nor on one of the following pages. I checked it with 3 different browsers (Chrome logged-in; Explorer logged-out but having a search history; Opera logged-out cookies and history cleaned) with slightly different results but on all SERPS the page is missing.
Now the interesting point: If I change the number of results per page from (default) ten to one hundred the page re-appear. Moreover, if I change it back to 10 results per page it is still in the SERPs. And if I now reload the results on the two other browsers (where I didn't made any changes at all) the page re-appear! (This probably means that the filter based on the IP or geolocation.)
I'm seeing this behavior for months on different computers. The problem/filter appears from time to time but not daily.
Is anyone else seeing the same thing? And is anyone having a better explanation than that this is a bug?
From a normal filter I would expect it doesn't change every few days (I would expect that the filter is applied permanently). But if you're right and it's a feature then they are trying to reduce the search engine traffic without penalize the page.