Personalize search... Check if you got "View Customization" somewhere on top left hand of your browsers
Maybe your login with any google program like youtube, gmail, adsense, etc.. It could be one of the reason why it have different search results.
yes this probably the reason. i notice this too when i'm login in google and search my keyword and surprise my site is number one log out and try again.
I never see these kind of problem. In my view it may happen due to the cached problem. Sumtimes its happen that a particular page is being saved in a definite browser. So after changing in the ranking its still remain same. On that time this kind of things may happen.
ip based. Your ie show your country and it is used by google to show the result. It is the same as you using google local eg: google.co.uk
There are tons of different datacenters you are just getting a different one. Sometime with the same browser if you perform a search a few minutes later you can get a different one as well.