would be interesting to see ouput from "symcfg show pool -sid 1234 -pool -in -detail -gb" , specifically Total Tracks, Pool Allocated Tracks and Total Written Tracks for those symdevs.

I believe earlier datastore might have been used while creating virtual machines

and later they might have deleted virtual machine(with virtual disks) or migrated virtual machine with sVMotion.

So from thin device perspective it is 90% allocated and from vSphere client perspective, it 50% used.