indyank - 4:03 am on May 19, 2011 (gmt 0)
Robert, I think you are spot on...You have very well summarized what I am seeing too and this has been happening for some time now.I initially saw a set of pages being pushed up for some keywords and these have now been replaced by another set of pages from different domains.
But google does keep a few domains intact on the front page, while the others are pushed into oblivion.However, there is a constant churn in positions.
what i am not certain is whether they are testing the heuristics for the document level classifiers or they have picked up a different set of niches (or keywords) to run this UE test.
If you remember backdraft7 and ohno were reporting traffic throttling and fluctuations a few months ago.But they don't seem to be complaining here recently :) Might be that google is done with evaluation of their niches.