There are probably two things that could be at issue.

1) Custom search now will alter your search results if you are logged in with a google account (username shows in the upper right hand corner.) So, to see what the rest of the world sees you should make sure you are logged out (or that your google profile is not customizing your search results based on usage history.)

2) Google has many different datacenters and they do not necessarily respond exactly the same all the time. Updates ripple through their databases and so you may see different results depending on which datacenter is being pulled up. If you google for "google datacenter comparison" you'll likely find a few sites that can do comparison searches.

The customized search above may take effect whether or not you've really tried to modify the search results for your profile. I recently discovered that google had just "remembered" that I visited one of my sites frequently from searches and so it gave that site a bit of a boost. I checked again from a non-logged in google page and saw the same site had dropped from 1st to ~5th.