The machine group information that is displayed is based on the machine group used to perform the most recent action on each machine. So it is possible for the machine group information to change.
If you then re-scan the first machine from a different machine group, the refreshed display will reflect this change:
The first machine is no longer listed with its original group because the most recent scan of the machine was initiated from a different machine group.
When agents check in with the console they will be listed with the machine group from which they were last scanned from the console.