Contributed By: Cliff Hobbs [MVP SMS]
Reproduced from the Microsoft Software Update Services: Frequently Asked Questions web page:

http://www.microsoft.com/windowsserversystem/sus/susfaq.mspx#top

"This most often occurs because the detection criteria for an update has changed—not because the update itself, or the update binaries, have changed. If the update itself ever changes (binary change), that would be documented in the knowledge base (KB) article associated with the update."