As you may know, Google search results change regularly as we update Google index. When we add new sites and incorporate updates to the content of existing pages, pages in Google search results shift. Some will be ranked higher than before for a particular keyword and others lower.
These changes are typically automated. However, Google reserves the right to make the ultimate determination as to whether a website's ranking increases, decreases, or remains the same. It is certainly google intent to represent the content of the internet fairly and accurately. To learn more about how Google determines sites' positions in google search results, please see http://www.google.com/technology/index.html
Occasionally, fluctuation in ranking could be due to differences in google data centers. When you perform a Google search, your query is sent to a Google data center in order to retrieve search results. There are numerous data centers, and many factors (such as geographic location and search traffic) determine where a query is sent. Because not all of google data centers are updated simultaneously, it's possible to see slightly different search results depending on which data center handles your query.
While we can't guarantee that any page will rank consistently in google search results, or be included at all for that matter, google webmaster guidelines offer helpful tips for maintaining a Google-friendly site. In general, webmasters can improve a site's visibility in google search results by increasing the number of high-quality sites that link to it.