From the site’s FAQ:
Whenever you visit Google and enter a search term or phrase, Google’s system must decide which sites to display first. Some terms appear in Google’s database hundreds of millions of times, but the first page of results displays only ten websites. Google works very hard to ensure that the top results are the most relavent to your query.
Google, along with other leading search engines, rely on complex algorithms to determine which websites to list first; they tweak their systems daily. As a result, the rankings fluctuate, and that is where RankPulse comes in – we track and chart the daily fluctuations of 1,000 keyword searches at Google.
Search engines are amazing gateways to virtually unlimited content, and at RankPulse our goal is to provide some insights into how they work. One of our primary intentions is to show webmasters that although unscrupulous search engine optimization (SEO) techniques may yield temporary benefits, they are not foundations for a sound and sustainable website……
Google, along with other leading search engines, rely on complex algorithms to determine which websites to list first; they tweak their systems daily. As a result, the rankings fluctuate, and that is where RankPulse comes in – we track and chart the daily fluctuations of 1,000 keyword searches at Google…..
(Thanks again, Philipp)