When viewing the "System Quota" numbers from your bandwidth breakdown page, you may determine that the numbers don't appear to be correct.

System Quotas are too low

If your system quota are too low, meaning that the sum of the domain files and email files are more than the system quotas... or the System Quotas or 0.0, then it would imply that the qutoas are not correct setup or are not working correctly.
For that case you'd go through this debug guide:
http://help.directadmin.com/item.php?id=42

and if you do require techincal support, include the output from all of the commands at the bottom of the guide in your support request.

Sytem Quotas are too high

If your can't seem to figure out how the system qutoas are coming to the number that it is, meaning you've counted all files under /home/user, and are just not getting the number to be that high, that's when you'd need to go hunting in your system to find out where those files are.  For that case, you'd use the "find" command to search the entire partition holding the User data to show you where all User files exist, and anything outside of /home/user is probably what's causing the confusion.
http://help.directadmin.com/item.php?id=39