Martin, I am not sure why Google would do that what you suggest. Don't get me wrong. I am NOT a G lover. Power corrupts and absolute power corrupts absolutely. I just don't understand why Google would take sites that are borderline Panda penalty sites and drop them down and at the same time raise garbage sites in their place. That has no benefit to Google.
To me, what makes more sense is that these times are times where their AI is learning. It seems to me that in order for an AI to learn, it would have to follow the same process as we do on a smaller scale when we do A/B testing. The baseline is Monday through Thursday afternoon, then it tests a different metric to see what happens to the serps, evaluates that change based on their own on page analytics i.e. immediate bounce backs, how many serps people click before stopping, how many pages deep they go before stopping ect. It goes back and forth in these tests until the Goldilocks AI finds the bed that is just right where there is an improvement over the baseline. Then Google announces another update to their algo named after a black and white furry creature (a skunk by any other name), and webmasters run around crying the sky is falling.
In the end, this all benefits Google. They believe that they will end up with better serp results in their baseline, they limit their corporate liability by being able to blame a computer program rather than direct manipulation to head off lawsuits, and they end up with an algorithm that is not able to be gamed. That I believe is their goal.