Reno - 2:48 pm on Aug 10, 2010 (gmt 0)
Google is now nothing like the fair minded upstart that bit by bit took the SE world by storm with good quality search engine results from a position of being the underdog.
As many here know, when you have a company that employs highly paid employees, they have to do something to justify their salaries. They can't just sit around and say "everything is working fine so we won't do anything". A few weeks of that and their jobs are in jeopardy.
So in Google's case, the highly paid and much in demand engineering staff is going to keep messing with the motor, because if they don't then they'll be perceived as not earning the Big Buck$. Sometimes they'll get it close to right and sometimes they'll screw it up. Their primary concern is to be busy -- if they get it right, then they can high five, but if not, that's a darn shame but hey, at least they were doing something. In fact, if the motor does get a little out of tune, then they have to try to fix it, which keeps them even busier. So it's win-win for them, and for everyone else, it's hit & miss.
Really, a relatively small handful of people are effecting us all, and if you see them deliver a beautiful & elegant set of SERPs, don't celebrate -- it's just a matter of time before they'll try to re-tune it, and that knocking you'll hear will be the valves out of position..... yet again.