AlyssaS - 2:42 pm on Sep 15, 2010 (gmt 0)
Well, google is officially mental. I have gone from 3rd to 4th to 9th to 8th to 6th to 5th to 4th to 5 to 3rd to 2nd in one week for my main SERP.
I think what's happening is that they are rotating databases, and they arn't in sync.
I made a major change last week, and have been monitoring cache dates to see whether Google has picked it up - and the cache keeps jumping. One day it says 7 days, the very next day, cache says 14 days - which makes no sense unless they are using different databases, some of which have more recent caches than others. If the caches are out of sync, the rankings will be too - as they will have been calculated on different dates.
So I think they are rotating databases, and they are all out of sync. I think this is a new development. I'm pretty sure that previously, you just got the database nearest you, which meant that the rankings you got were consistant, as they came from the same dataset. No doubt someone will correct me if I'm wrong.