I've been making some noise (often sarcastically) about Google not acknowledging the SERPs changes that webmasters have been seeing over the last few weeks. See comments here, here and here. Basically, Matt Cutts has been saying repeatedly that the changes lots of people are seeing in the SERPs is just "normal operations". I guess he got tired of hearing from people like me, so he posted today on his blog and clarified that the changes we've been noticing over the last few weeks involves a data refresh - an index update - that is occurring on a daily basis. This is the same data refresh that had impacts on June 27th, July 27th, and August 17th.
So, these data refreshes (or index updates) involve one particular set of data, and we aren't told what that particular set is. Obviously, there is other data that is refreshed periodically as well, but those apparently never get noticed by the public. This particular set of data is now getting refreshed often, and at the moment at least, it appears to affect a subset of webmasters every week or so. In other words, this week, it may cause turmoil for X number of webmasters; next week, it causes turmoil for another group of webmasters, and so on. So every few days, we're likely to see a new forum thread of people wondering what the heck happened. And the standard reply to that will be that "nothing major happened".
Ok, nothing major - in the sense of an algo change - happened, but major changes can be seen by whichever group has been affected that day. So, my feelings back on December 10th, when I ranted that data refreshes suck, still stands. My personal belief is that this particular set of data, that is now being refreshed much more often, is faulty in some way (my best guess is old data sneaking in). And because it is considered normal daily operations by the Googlers, they are probably no longer watching and evaluating the changes it makes.
With a major algo change, Google pays attention to the cries of "foul" from webmasters, and often sets up a special email address so that webmasters can send in observations. But since this data refresh is not major in the eyes of Google, I think the problems it is causing is going unnoticed by Google.
Obviously, I have no doubt that Googlers would poo-poo much of the above, and dismiss my thoughts out of hand, but I can only see what I see, and think what I think. And those are my observations and thoughts. Take 'em or leave 'em.