That's because search engines are tuned for optimized results, using cookies and other tools to harvest information from your browser and its search history to provide personalized results that are hopefully relevant to you. Sometimes that's good and saves time, and sometimes it could feel a little creepy. Even more importantly, when Google and other search engines are trying to personalize results, they may not be delivering all the relevant hits, in favor of customizing the results to previous searches and eliminating results that could be more relevant.
If this subject intrigues you, I recommend reading an article written by Mary Ellen Bates, a professional researcher, who knows (and regularly shares) all the tricks of searching - Getting "Pure" Search Results. A couple years ago I attended a workshop that she taught, and came away with my head so full of useful information I thought I would burst.
My personal favorites cited are Scroogle and Ixquick (a metasearch engine that employs the results of several other search utilities). Ms Bates also recommends the Chrome browser's "Incognito mode." (I'll have to try that). ECRL's automation services department regularly recommends Google's Chrome browser for its unencumbered light-weight speed.
Scroogle - http://www.scroogle.orgHappy searching!
Ixquick - http://www.ixquick.com
Google Chrome this is a browser you download. Using Chrome itself does not result in an anonymous search. After it's installed, go to the wrench icon in the upper right-hand corner of the window and select from the drop down menu options "new incognito window."
Barbara Misselt, Director