The provisioned space includes swap files and snapshots. So you may have some VM or files residing into your VMFS not listed on this view.
Q1: This datastore is only used by this ESX/vCenter? If not, there may be ESX servers accessing it and storing VMs that does not belong to the same VC you are in.
Q2: Have you verified on the browse datastore if you don't have any folder not corresponding to the VMs listed?
By command line, you can also check what are the sizes of each folder by doing:
du -h -d 1
Let us know how it goes.