Google uses more than 100,000 servers to store its data. These servers build-the Google data centers. Depending on the time and the place from which you access Google, Google will redirect you to different data centers.
For example, you might be redirected to the IP address 64.233.161.105 or to 216.239.57.105 when you enter www.Google.com in your web browser. Both IP addresses reflect different data centers in which Google will search for your query (the IP addresses in this example were operational on 29 March 2005, they can change at any time).

There are very many different Google data centers and each data center has its own index. Normally, the contents of the different data centers is about the same.
Webmasters who observe several data centers have found out that there are currently three types of data centers. Depending on which data center group you use for your search query, you’ll get different results.
What does that mean?

It seems that Google uses different algorithms in these data centers and that Google uses some data centers to test algorithm changes. Using only a few data centers allows Google to test algorithm changes without affecting all search results.
The three different data center groups that have been observed by some webmasters probably reflect the test of different algorithms that affect the same ranking factor.

Data center group one (the biggest) uses the current algorithm, group two (a small group) tests algorithm change A, group three (another small group) tests algorithm change B. That way, Google can test the best way to improve its algorithm without affecting all search results.

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