Question on df

We're running Tru64 Unix V5.1B pk4. Lately we've been seeing a problem
where the 'df' output is incorrect. It might show we're at 94%, but in
looking at the actual disk consumption, it should be something quite
different. (Typically much lower.) Typically the disks we're having
problems with are RAID sets of fairly large size. (300GB or higher.)

We've talked to HP and they suggested:

Use the 'quotacheck' command to fix it. Sometimes this seems to help;
otherwise it doesn't.
Since we're using AdvFS, we should be using the 'vdf' command to check
on disk space. This has proven of no help at all. It reports the same
(incorrect) info as 'df'.

Anyone seen this? Is it maybe an O/S bug? 

Much thanks,

Chris