When ranking your pages, Google looks at factors outside of your site as well. Here are some of the key ones:
The number of linking domains. The number of domains linking to you is one of the most important ranking factors.
The number of linking pages. There might be a number of links from a particular domain to your site, their number is a ranking factor too. However, it is still better to have more links from individual domains rather than from a single domain.
PageRank of linking page. Not all pages are equal. Links on pages with higher PageRank will be a bigger factor than those on low PR pages. Therefore, you should strive to build links from high PR pages.
Link relevancy. Some SEOs believe that links from pages related to your pages topic carry more relevancy for search engines.
Authority of linking domain. Similarly to a page PR, the authority of a domain may be a ranking factor too. For that reason, a link from low PR page on a high PR site will be worth more that from a lower PR one.
Links from homepage. Similarly, some SEOs believe that links from a home page of a linking domain carry more strength than those on one of its pages.