For every query performed online, the search engine selects from millions of possible results and displays the most relevant for the topic in question. The pages with the highest ranking are those that distinguish between the various results. And everybody strives to increase search engine rankings and thus be displayed among the first.
Past experience allows search engines to rank websites by determining relevance in relation with a number of elements or factors such as one way links. What people are interested in when trying to increase search engine rankings is to use the algorithms or the ranking rules in their favor. The ranking is achieved by mathematically combining several algorithms, but we don't know which. This is a very closely-kept trade secret. If people were familiar with such elements, web masters would be able to create websites that correspond exactly to the demands of the search engine, even if they do not match the query criteria.
Yet, from direct experience and common practice we can say that keyword density for a website, influences PR. You can thus increase search engine rankings by achieving good search engine optimization. The location of the keywords on the page is very important too, in addition to the frequency. When the keywords are present in the first paragraphs of a certain article and sprinkled in the body, but not too frequently, the search engine will consider the content suitable.
The page ranking differs with every search engine because each uses a specific combination of algorithms for indexation. Your effort to increase search engine rankings should show results on the long term, progressively, but you need to constantly work and improve the way things stand for you. Do not care too much about the differences between the PR given by various SE; you'd better focus on effective link building strategies to improve these rankings.
In the attempt to increase search engine rankings rapidly, some marketers make the mistake of spamming the search engines. This is the case when the same word is repeated hundreds of times on the same page. Search engines have very advanced methods to search for this kind of spamming. Users could also file complaints against such messages, which ruins your business reputation. Stay away from trouble by using only approved methods. Why risk the future of your business by trying to increase search engine rankings with dishonest one way link building strategies?