This is a common bug on cPanel 11 systems, where you do an account listing through Webhost manager or try to set a user’s quota and everyone is showing as using 0mb out of unlimited. The most common fix for this is to run:
/scripts/fixquotas
This will re-initialize the quota system and usually fix the error.
On VPS servers running Virtuozzo, a kernel bug in earlier versions of CentOS and Redhat have been attributed to this error (and similar errors) as well.
/scripts/fixquotas is great, but I recommend running it during an offpeak time, like the middle of the night if you are running a server with a lot of clients on it. It can be pretty resource intensive.
Yes, You are correct.