I think G's lack of correspondance is more related to the reality of the situation. Every change they make will positively affect some webmasters and negatively affect others. They know this and they expect webmasters to know and accept this. I think it is a reasonable expectation. Which is why I don't track SERPS anymore. I just focus on maximizing exposure of my content. Focus on what you can control. Don't let fluctuating SERPS distract you from improving your core business/service.