SEO Guru's say: on the one hand, search engines periodically change algorithms, when search engines change algorithms the rankings also changes position. The position of keywords could improve or move down in prominence. Whether each page goes up or down in the results depends on the changes that have been made to the search engine's algorithm.

On the other hand, top position in search engines last until a competitor decides to optimize a page from his website with the same keywords. If the competitor succeeds then the existing top page slips down to next position. In such a situation the entire SEO process needs a review and redone with fresh content for regaining earlier position.