Please don't believe anything that I say in here, because this is only my experience.
I am a web designer a SEO. The problem is your browser. Well, it is not really a problem is just if you search your website in one browser for a special keyword and you just keep clicking that website, some browsers will think that you really like that website and they make it appear on a different position. For instance, if I look on firefox or IE 7, the keyword "my website" and the url "www.mywebsite.com" appears in one position, and if I keep clicking that website my browsers will think, well we think that this guy like this web, so we are going to move it a little bit up. I think this is a setting you can manipulate on you browser.