Google personalizes results based on your unique search history.
Let's say your target keyword is "cheap books".
#1: You search for "cheap books" in Firefox. Your website is ranked #5 on Google.
#2: You then sign in to your Gmail / Google account still in Firefox. You search again. Your website is ranked #20 on Google.
#3: You then switch to Internet Explorer. You search again. Your website is ranked #10 in Google.
Google looked at your search history and tried to personalize the results for you. In #1, it looked at the searches you have made recently in Firefox. In #2, it looked at the searches you have made recently while logged into your account. In #3, it looked at the searches you have made recently while in Internet Explorer.
If these search histories are different (which they undoubtedly are), Google may overpersonalize your results.
What does it mean to "personalize" results?
Continuing the example of "cheap books", let's say that recently, you were searching for "cheap books about dogs". Next time you search for just "cheap books", Google boosts the ranking of sites about cheap books about dogs, because it knows you have been interested in these recently. Thus, your website about generic cheap books will fall in ranking in comparison to these dog-related cheap book sites, even though you weren't trying to search for them.
But to someone who hadn't been recently searching about anything relating to cheap books, your site would be higher in the rankings.
How do I check my REAL rank then?
Using Google Chrome, open an incognito window (click the 'tool' button in the top right corner, then click 'new incognito window'). An incognito window is designed to have no access to your browser's history information. Do not log into your Gmail/Google account. Go straight to Google, and search for your keywords. This is your real rank, because it is a non-personalized search.
If you don't want to get Chrome, you can also just log out of your Google account and clear your browser's history and cache, but this method has the side effect of.. well, clearing your browser's history and cache. :p