You're doing a little SEO for web research and you search for your own website on Google. On this day, your site is sitting happily at the #1 spot. You check a few weeks later, preforming the same exact search and yet, your site rank has dropped a few spots! Your head starts spinning and you don't understand why this has happened. You haven't made any changes to your site from when you last searched, and yet your pagerank has dropped. This occurrence can be summed up in 4 reasons:
1. +/- of Linked Pages: If the number of sites linking to your own site changes, this will factor in determining what sites are indexed by Google. Should the number go down, you can assume the same will happen to your page rank. Think of sites linking to your own as a vote. The more votes you get, the better!
2. Google Personalization. ( https://support.google.com/accounts/answer/54068?hl=en) Google watches what you click on in the search results page. From this, Google is learning what your site preferences are. For example, if you preform several searches, with almost all ending with a click on a search result from ebay, Google learns or assumes that you really like ebay. This gives ebay a boost in ranks and you'll start seeing more listings from ebay. This will hold true to the search results of your site.
If you prefer to, you can turn off Google's personalization feature. Here's how: http://support.google.com/websearch/bin/answer.py?hl=en&answer=2410479. This will help in yielding results that aren't based on personal content.
3. Data Center Accessed: Google uses thousands upon thousands of data centers and the data center you accessed today will probably be different than the one you accessed yesterday. This is due to load balancing efforts.
4. Localization: Say you happen to be accessing the same data center as someone who lives 4 states away from you, your search results could STILL vary from this persons results due to localization. This is Google trying to further "personalize" your search results.
And with Google page ranks, and general SEO, you should always keep in mind these two things:
- Frequency of Keywords: If a keyword only appears once on a page, it will receive a low score.
- Number of sites that link to the page: Google determines relevance of a site based on how many web pages are linked to the page in question.
SEO success comes with great patience and the ability to take on new strategies, so don't fret too much if your ranking fluctuates every so often, it's natural due to the above factors!