Google recently posted the video above to advise people about personalised search results. They are now serving personalised search results whether you are logged on or not, unless you TURN IT OFF.
Overall this has to be a good thing, tailored results should mean more relevant results and a better search experience. However, if you are monitoring your site's performance in the search results personalised results can cause premature popping of champagne corks.
Several times recently I've read posts in forums along the lines of, "When I search using my PC I'm number one in Google but when I used my friends PC I'm nowhere......what's wrong with my friends PC?"
Well, nice try blaming your friends PC but the truth is Google is flattering to deceive. Because you visit your site a lot Google's personalised search results are favouring it.
Follow the advice in the video above and you'll be fine.