Google updates their search algorithms and indexes fairly often. Results change all the time. The goal for Google is to make this transparent for searchers by providing relevant results. But when a change happens, typically the webmaster will notice and may be impacted strongly by the update. So what are the Google processes and thoughts behind an update?
Google's JohnMu has an interesting post at a Google Groups thread in response to a webmaster being upset with a recent change in which results Google is showing for a particular term. In JohnMu's response, he explains why Google makes changes, for how long the changes may appear and so on. Let me explain that we do not learn much about how the changes work technically, but we do learn the thought process behind Google making these changes.
The premise behind all Google algorithm and index updates are to "improve the users' experience on our [Google's] sites." Google's "engineers are constantly working on" that, day in and day out. So Google is often trying "new things and run experiments" to collect data to help Google come up with new ideas on how to "improve" the search experience.
John explains that these "experiments" and "new things" can "run for a day or for many months." The thing is, these experiments "generally evolve over time," so although the experiment may do one thing, it can evolve to do other things (hence the constant Google flux). These changes to the experiments are "mainly based on that data" Google collects as they run tests.
Google is constantly changing their algorithm because "world around us is changing rapidly, our users' expectations are changing equally," as John explains. John adds that "sometimes," these experiments can "lead to changes which not everyone likes." "Not every site can be listed in all search results, or even in the top 10," John adds.
I personally don't have much to add to this. It all makes logical sense, but I thought it would be good to document and have for later.
Forum discussion at Google Groups.