Hi,
out of a fisrt impulse, I would suspect because Isilon is lying.
And that should be fine as long as you know about it. It is after all the basis for "thin provisioning".
Can you verify the capacities you are seeing in OCI are the same the the native storage management utilities report?
When it comes to capacities, OCI basically takes the information received for a particular object and showes it to you.
So if a single internal volume has twice the size of the capacities of all the disks, it's becuase it's reported as such by the array.
And who knows? Maybe the array has some magic compression capabilities to allow storing of that much data.
Regards,
Gerhard