An artifact is currently associated to a Milestone, or contains other attributes that are not present when you view the ‘Revision History’ data for that artifact. Why is this?
The Milestone, other data or attributes were created at the time that the artifact was created. They do not specifically show in the Revision History as they are all part of the 'Original Revision'. Note that the revision history of the Milestone as well as the scheduled items for that Milestone will show the association details to the artifact.