tristanperry - 3:05 pm on Sep 15, 2010 (gmt 0)
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.
I agree, I've noticed this too.
It also happens with me when I check (via the site: operator) how many of my site's pages are indexed. Sometimes I'll check a few times in a day and get slightly different sets of results each time. (And Google Webmaster Tools also reports a completely different number of indexed pages to the actual SERPs, too).