My two cents are: don’t try and over think it on the micro scale. I’ve had clients who go nuts until they adjust to a macro view of their website and look for healthy trends (more links, more involvement, more traffic, overall higher placement month to month, etc). None of us are privy to the thousands of algorithmic decisions Google goes through to provide what comes up in a search. What we do know is pretty consistent though. Just get the basics right continually and everything works pretty well.
Don’t forget that not every search is the same (far from it). I work with a large insurance company and we’ll do a set of searches to see where their sites show up and all of the sites can literally be in completely difference places for the 5 or 6 people that do searches, sometimes even a whole page back or forward.
And actually thought of something else: As I assume you have analytics, check around there how your visitors find you. It might be that as JoshWhite is saying, that you’re overthinking it on the micro scale.