To explain this, you should understand that search engines use more than just the search terms when providing results for search queries. Google and other search engines have a record of words that are semantically related to search words, terms or phrases. They also have a record of the words used on your page. Search engines will first identify all pages in their database that match the search terms. They will then compare the word usage record of the keyword related pages to the semantically related words. Pages that use multiple words and synonyms related to the keyword or phrase in the content are establishing a Preponderance of Relevant Content. When all other things are equal, web pages that do this the best are placed highest in the search results. There are a few other factors but this explains how a web page with a low Page Rank can beat a web page with a higher Page Rank in the search results. And you should use google api ranking checker to determine your good and bad points.
Google provides a free tool at http://labs.google.com/sets. This tool will generate a set of words semantically related to your keywords or phrases. To generate a list of semantically related words simply type in your keyword and 1 or 2 additional words that relate to that keyword. The set of words in the generated list can be used in your content to provide a Preponderance of Relevant Content for your site or blog.
When creating content for your blog or website use synonyms and semantically related words to boost the power of your keywords or phrases. Over a period of time, Google will learn that your content just doesn’t contain exact keywords but also is an authority site providing quality, relevant information to your visitors. This after all is what search engines want to do and it may be the advantage you need to overtake your competitors in the SERP wars.