What Does Google Update Do?

What are the reasons behind the Google PageRank update on 27th June, 2011?

• On 27th June, 2011 Google rolled-out PageRank update for the websites around the world. When I was checking for the big brands PageRank, I found that some of the websites' PR went down even though they have been placing only fresh and unique information which the community readers enjoy reading AND they are following Google guidelines also. According to you, what are the reasons for this drop for biggest brands who have good reputation. I understand that there is no solid answer for this question, But if someone can share their thoughts, It would be great.

• Answer:

  PageRank has nothing to do with fresh and unique information. In my opinion, "toolbar" PageRank is probably still calculated roughly similarly to the description of PageRank in the original "anatomy of a search engine" page. If that's the case, then toolbar PageRank measures only raw "link juice" - quantity and strength of in-bound links. (where link strength is only measured by number of links pointing to the linking page.) All other SEO factors are completely ignored in this measurement. So if certain websites dropped in toolbar PR, it has only to do with the raw strength of their inbound links being less than it was previously. A few other notes - "toolbar" PageRank updates are just updates to a green bar. They don't correspond to rankings changes. Google calculates link values in real-time, and then pushes out a toolbar update whenever they feel like it. And, overall toolbar PageRank is basically a worthless metric for anything other than very quickly sizing up a website. Also - maybe there's just something strange about this particular update, since I'm seeing http://google.com having a PR of 9...