Dynamics CRM Task List gets Out of Sync

Hi All,

Have a strange issue where it seems 1 in 40 or so tasks appears to get out of sync in Dynamics CRM. It is CRM 2013 and the user is configured to use the synchronization method 'Microsoft Dynamics CRM for Outlook'

I'm not sure what to look at next, the users are only closing tasks from the CRM so it cant be getting confused between Outlook client changes. Any ideas?

Thanks