This column can sometimes cause confusion if it is seen as an "Agent Last Connected" time. But there are specific agent events that cause this column to be updated.
Events that cause the Last Update Time to refresh are
*A policy must be directly updated, updates to user groups will not cause an update event.
For those interested in an "agent last checked in" field there is an open enhancement request PM-2443.
You may want to review: Article ID: 170468 DLP Agent status not reporting as expected on Enforce