[CentOS] XFS quota usage mismatch on CentOS 6 ?

We have a number of home directory servers running CentOS 6 exporting XFS file systems over NFSv3

We enable user quotas for all users - with a relatively small allocation to each user

Every now and then, we get a user that has a quota mismatch - where they are (usually) over their quota, but their actual usage (via du) is significantly less

This happens on any of the servers

I suspect that the mismatch is probably due to file space being allocated on the server, but somehow not referenced by a file - may be by deleted files?, but lsof on the client doesn't show any suitable deleted files

The 'problem' usually resolves itself in a day or so - although I have no idea how or why etc ...

Has anyone seen an issue like this - or know of any XFS tools that could be run on the server to find and fix the missing quota allocation?

Thanks

James Pearson