identifierForVendor changes when updating an app

The identifierForVendor is not supposed to change unless all apps by the vendor are removed from the device. What happens if I have a single application that is updated. Can the identifierForVendor change? Another way of formulating this question is: Is an app upgrade the same as an uninstall and install (at least in some sense)?

The reason I ask is that if I install the current version of an app from the App Store, and then "Archive" a new version via XCode and install it over iTunes the identifierForVendor changes. I don't know if this is a special case or if this is also the case with a "normal" App update/upgrade.