Here is a great explanation by William Slawski of various ways that Search Engine may rerank search results. Conclusion – what you may be seeing is different than somebody else, even under the same search key. If you have hired a SEO professional to optimize your site, ask them for 6-different ways that Search Engines can rank your site – this will tell you whether they actually know what they are talking about.
For a full explanation of each of the methods below, please visit William’s web site and the post “20-Ways Search Engines May Rerank Search Results“
Search engines try to match words used in queries with words found on pages or in links pointing to those pages when providing search results. Often, the order that pages are returned to a searcher are based upon an indexing of text on those pages, text in links pointing to those pages, and some measure of importance based upon link popularity. Before pages are served to a viewer, however, they may be reranked for one reason or another. Here are some possibilities:
1. Filtering of duplicate, or near duplicate, content
2. Removing multiple relevant pages from the same site
3. Based upon personal interests
4. Reranking based upon local inter-connectivity
5. Sorting for country specific results
6. Sorting for language specific results
7. Looking at population or audience segmentation information
8. Reranking based upon historical data
9. Reordering based upon topic familiarity
10. Changing orders based upon commercial intent
11. Reranking and removing results based upon mobile device friendliness
12. Reranking based upon accessibility
13. Reranking based upon editorial content
14. Reranking based upon additional terms (boosting) and comparing text similarity
15. Reordering based upon implicit feedback from user activities and click-throughs
16. Reranking based upon community endorsement
17. Reranking based upon information redundancy
18. Reranking based upon storylines
19. Reranking by looking at blogs, news, and web pages as infectious disease
20. Reranking based upon conceptually related information including time-based and use-based factors
Conclusion
The rankings that you see for web pages in response to a query may not be the same rankings that other people see.
Great explanation of the various method – very nice job by William – thanks for the posting.