the truth is, Google's SERP is constantly changing everyday, so better update/optimize your site everyday!..
One of my site,rank much better than before,I think google SERPs are updating these days.,Hope more traffic from google.
i also track 50+ Keywords everyday & i find that they continuously change their positions. So i am agree with thel all who said SERP's change everyday.
Jim is right, when i m tracking keywords every day for my websites, i daily found a little bit difference in the position for each and every keywords, some times, it does well and some time it goes down, but the difference would be (Plus or Minus) 3 . Not more than that for any keyword.