Why Are Google Search Results Different?

Why are Google search results different from computer to computer?  This is a fairly common questions that that comes up concerning Google search for specific keyword phrase rankings.
A few years back, if I looked at a search engine result page (SERP) on my computer and you looked at yours, the results would be the same. Now, it almost guaranteed that we will not have the same results.

There are many factors that are currently involved that make it difficult to demonstrate specific keyword ranking positions. Google Calls this “personalized results”.
The following are the main factors that affect how search results are displayed from computer to computer. The results can be incredibly dynamic.

Every computer has a unique Local IP and Mac address. Google determines your approximate location, examines your machine cache, online history and tries to deliver appropriate results based on:

• Web history – Local Browser history, Browser cookies and online browsing history search results.
• Whether you are logged into of logged out of anything Google. However, Google will still deliver personalized results.
• Recent searches – Uses browser cache, Local Browser history and online browsing history
• Universal search – This is a way Google Blends results from other parts of Google such as YouTube, Images, news, etc. into search
• Geographic location – Search will vary by location
• Social search – Essentially blending of social results in the SERP.
• Google Data centers – Google has many data centers. Results may vary from Data center to Data center.
The result of personalized search are not always beneficial to the user.
Quoted from Wikipedia and credited below.

“Several concerns have been brought up regarding the feature. It decreases the likelihood of finding new information, since it biases search results towards what the user has already found. It also introduces some privacy problems, since a user may not be aware that their search results are personalized for them, and it affects the search results of other people who use the same computer (unless they are logged in as a different user). The feature also has profound effects on the search engine optimization (SEO) industry, since search results are not ranked the same way for every user – thus making it more difficult to identify the effects of SEO efforts.[11] Personalization makes search experience inconsistent for different users requiring the SEO industry to be aware of both personalized and non-personalized search results to get an increase in ranking.[10]
Personalized search suffers from creating an abundance of background noise to search results. This can be seen as the carry-over effect where one search is performed followed by a subsequent search. The second search is influenced by the first search if a timeout period is not set at a high enough threshold. ”
References:
10, 11: http://en.wikipedia.org/wiki/Google_Personalized_Search