The retention categories are updated during the run of the archive task or if you do a run now of the archive task and choose shortcut processing.
The Moved Items tab on the policy controls if the Retention Category is updated or not.
Have read of this Technote:
http://seer.entsupport.symantec.com/docs/327537.htm
After an archiving and shortcut processing run retention categories for items where the Retention Category was selected by the user or set by a custom filter are reverted to the retention category of the folder the item is in. This can be witnessed in Archive Explorer.
Details:
To resolve this issue perform the following:
1) Select the properties of the Mailbox policy for the affected user.
2) Select the "Moved Items" tab.
3) Deselect "Include items with Retention Category selected by the user or set by a custom filter.
4) Synchronize the users mailbox and test again.
If you want those items to be updated make sure Include Items with Retention Category selected by the user or set by a custom filter is selected.