The changing fortunes of being the last program on the Start menu
One of the thing I didn’t mention during my discussion lo many years ago on how Windows XP chooses what goes on the front page of the Start menu is that the the last-place position is actually kind of special.
In Windows Vista, the last-place position was used to hold the program you ran most recently, regardless of how often you ran it. In a sense, the last slot is a one-entry MRU. (This was the general idea, although it could be overridden by other principles. For example, if the program you last ran is pinned, then it shows up in its natural pinned location rather than being shoved to the bottom of the Start menu.)
In Windows 7, the magical last-place position goes to the application you just installed, if one exists, to save you the trouble of hunting for it. (As we saw some time ago, you can set the System.AppUserModel.ExcludeFromShowInNewInstall property to remove your shortcut from consideration as a newly-installed program.)