Things are simple.
Each search engine has it is own script - algorithm. It is said that Google uses an algorithm with over 100 criteria in order to put the sites in first place.
For example:
We search at Google "food". Results are over 1,00,000,000. The first site (foodnetwork.com right now) is too nice for these Google criteria. The second site (latimes.com) is the same but 1 criteria is bad.
We do not know which criteria of foodnetwork.com brings it in front of latimes.com. It may be the visitors, the direct clicks, nobody knows!
The point:
We have to watch our criteria in order to complete them according to the previous and next sites positioned.
