Is is a big differenence or a small difference? What %?
You may be leaving out swap files, log files(which should not add up to much) as well as snapshots. Pick a lun or two where there is a discrepancy and use the datastore browser or an ssh session to do a careful inspection to find what files types are causing the discrepancy.
vdf -h shows space for each lun in the service console. Is that different that what your client/ vcenter server shows?
Are you using virtual center? Sometimes those numbers are slower to update than when you are connecting direct to host with client or using ssh.
One of the nice things about vSphere is it actually lists the date/time of when the last update to storage capacity was made.
