The Google search results that you see are frequently not the same other people see, especially if you’ve previously searched on the keyword phrase. Google currently provides “personalized search” results; they base the your search results on your previous searches and clicks. This occurs whether or not you’re signed-on to Google.
Here are two examples:
1. You are running a Google pay-per-click ad and you want to see the position of your ad. You check it frequently, but of course you do not click on it because you don’t want to incur the click costs. In this situation, Google will likely move your ad down. Since you’ve searched frequently and never clicked on your ad, Google will conclude that your ad is not what you are looking for and show you other ad at a higher position than yours.
2. You have an organic result and periodically check the position. You probably click on this occasionally to show Google that this is a popular result. Google will likely improve the results position, since this result is popular with you. I have seen results make their way to the top of the first page, when in fact the same result is on the second page for everyone else.
If you want to avoid Google’s personalization bias, here is a great plug-in provided by Yoast. It works with both Internet Explorer and FireFox. Once you install this plugin, you will have an additional search option, Google.com Unpersonalized. You can either set this as your default search engine or just select it when you want to see unbiased results. Either way, it’s a great tool for seeing where you really rank in the search engines.