Search engines score Web Site Design on a vast range of criteria, which are constantly assessed to ensure that the most suitable sites appear in your search results.
Although Google tries to think like a human being, it is still only a computer & so relies upon a complex algorithm to compile the necessary data. This algorithm rely's upon looking for over one hundred different factors on each page which it then scores in order to be able to rank each site. These factors can also be over done so it is very important to seek professional guidance before attempting to optimize a site yourself.