I've been testing this for two weeks, on different machines with different ISPs, and the results are always the same....#1 when logged in, #10+ when not logged in. Makes me wonder...does Google manipulate its organic results for known (paying) customers?
Try it for one of your target phrases and let me know if you see something similar.
I've seen this move other sites around, while my site stays in the same place - depending if I'm logged into Google Accounts or not.
So they don't just "manipulate" for paying AdWords customers, but anyone logged into Google Accounts.
There were posts from a couple years ago on how to disable personalized search, I'm not sure if it still works or not.