The way that the search engine companies have designed their algorithms is to in
effect mimic the way human's treat each other e.g., say someone applies for a job in a big company and for a very high post. The company uses a sort of formula in picking likely candidates and also sets out the basic requirements needed for the candidate e.g.,
(A)Good training to handle that post which can be compared to the Onpage factors and remember the Onpage factors are the changes you make to your site physically, this includes optimizing the Html design elements, optimizing internal link structure, and writing good contents that suits the sites needs etc
(B)Good qualifications, academic wise the candidate needs appropriate qualifications which again can be compared to Onpage factors.
(C)Other individual qualities the candidate should possess which can be compared to the more important Onpage factors. Some leading search engine optimizers who are well reputed in the industry often ask this question to themselves while optimizing a site. Why should a site rank higher among millions of other sites when they have the same words in their content? This is where a site needs to posses its own individual quality. So it stands out more when compared to the other millions of sites.
If it can do this then optimizing the site is much easier and it will rank higher without much effort.