Sgt_Kickaxe - 8:15 am on Jun 27, 2013 (gmt 0)
Let's simplify this.
Google now serves up customized results as often as they can so there is no set of "best" urls for everyone. This means that they need to monitor a larger number of sites for spam whereas before they were ok if a human simply looked over the top 10 sites for any given keyword, to start off.
By rolling out changes over a 10 day period they allow the human rating team to keep up with the spam which suddenly floats. Sure they algorithmically detect as much spam as they can but the final results will always be human evaluated, not even Google can get it right 100% of the time with a machine.
I suspect that after rolling out these changes there will be a quick 2nd update to adjust for changes in link values.
Also, don't forget about the changes over time patent which makes sites that deserve top spots take a long time to get there. We won't see the final results until well after they are done rolling out changes. Likewise they already started making changes a few days ago, perhaps even 10 days ago now judging by the spike in page crawling I had.