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There are two reasons for this.
The first is the so-called "filter bubble" phenomenon. Google delivers content ranked by an order of what they think you want to see. And there are presumably more factors taken into acccount than just your click or search history when you're logged in (cookies, geolocation, language settings, ISP, ... endless possibilities).
A simple explanation of this has been published by the Duckduckgo people at http://dontbubble.us in order to raise more awareness to this problem.
The second reason is obviously just an updated search index. The web content changes constantly and so does Google's search index. If you look at your search results a day later, you may also get different results.
I'd assume also third reason: the index and its results not being strictly deterministic of sorts - operating on a best effort basis within preset time delays, momentary availability of nearby resources, and such.