Google's index is always increasing and they re-evaluate page rankings of the web pages on more or less a monthly basis. It's known as the "Google Dance". The Google Dance can be seen through two other searchable servers (www2.google.com and www3.google.com) in addition to their main server (www.google.com). But, The search engine ranking results will be different during the google dance. Once the Dance will be over, the new ranking will be visible on all the three servers.