Hi ThompsG
No, in vCenter, datastore/monitor/performance or datastore/summary, shows that the datastore has the same amount of free space as df -h output.
I've migrated the 200Gb HDD on ESXI05P-Archive to ESXI01P-Archive datastore to see if it makes any difference.
ESXI01P-Archive is not being over allocated like ESXI05P-Archive.