I'd like to know if I am the only one seeing this. For my particular search keywords Pre-panda my site was at number 3. Post Panda the site fell to number 9. However, I have noticed that while I am logged into Gmail, Adsense or any other Google account my site shows at Pre-Panda - number 3. The instant I log out and check the search terms it's back to Post-Panda. I'm sure their is a reason for this, but I have yet to figure it out.
Well google shows user what they want to see.. Your serp depends on what you have clicked before, what sites you have visited and so on. To get trully unbiased SERP's, one should use Firefox private browsing or Google Chrome Incognito mode.
You can also log in to your Google Account's "Dashboard" (google.com/dashboard) and disable history. That neutralizes the personalization effect rather well, although geographic localization is still in play. To handle localization you need to make other efforts, such as searching via proxy servers or using the Adwords Preview Tool.
For people who need to track a large number of SERPs, there are paid services that make it their business to collect "neutral" data.