Mobile users sync every 20 minutes after choosing to manage the background sync settings as Never.
When you stop managing the background sync settings for mobile users in Workgroup Manager by choosing Never, it's important to understand that this does not mean syncing will stop altogether. Rather, it means that clients will no longer use the settings specified in Workgroup Manager.
Instead, they will revert to the user's settings, which are to sync every 20 minutes in the background by default. If you want to stop the mobile users from syncing their portable home directories in the background, manage the Background Sync settings as Always, and remove everything from the Include list.
This document describes Mac OS X Server 10.4 or later when used with Mac OS X 10.4 or later clients.