The reason why some search engine ranking positions fall or increase has everything to do with how Google interprets a search. Google is always evaluating and testing to make sure the searcher is provided with the most relevant search result.
All Searches Are Not Created Equal.
All searches are not created equal. Search results are only accurate at the time of search. My search result will look different from your search and will fluctuate because of Google’s continuing algorithm modifications, differences in our physical locations and personalization settings.
Real Time Data
Fluctuations in rankings may also occur due to real time data. Facebook, Blogs, Twitter and other real time content providers have motivated Google to index the internet in real time. Thus, Google may determine that a blog post, for instance, has significant value over a static web page and rank it higher.
Your State of Mind
Since Google is trying to provide the most relevant search result, they’ll try to provide search results based on what state of mind the search is in. For example, weather the searcher is in a ‘browsing’ or ‘buying’ mode at the time of search. Search results will determine this answer. When you see Google ranking Yelp and other directory websites within the search results, Google has determined, based on in-depth research, that the searcher may be browsing for goods and services rather than an immediate need to purchase, which would be indicated by a more specific product search or problem the searcher is having. For example: ‘washing machine repair companies’ (browsing) or ‘help with flooded basement’ (immediate need).
Other factors, with regards to fluctuating rankings, could be competitors using SEO techniques and new websites being indexed by Google.
Our job, as SEO specialists, is to monitor this activity, determine how Google is interpreting that search and modify our position accordingly by providing quality targeted keyword phrases and unique website content for Google to index.