When you delete a logical volume, the Oracle FS System reconditions the space (by writing a predefined bit pattern) before reclaiming the space for reuse. As the previously allocated capacity frees up, the capacity becomes available for allocation.
When you delete a large volume, the operation can take awhile for all of the capacity to be reclaimed. Because of the additional time that is needed to recondition the drives holding the volume data, the amount of physically allocated capacity plus the amount of physically free capacity in the owning drive groups might not equal the total physical capacity in those drive groups. Therefore, while the deleted capacity is being reconditioned, Oracle FS System Manager (GUI) displays the amount of physical capacity that remains to be reconditioned as “preparing.”