A minimum amount of free space in the selected Storage Domain is required to create a logical volume.
When you create a new volume, the actual amount of physical capacity that is consumed from the free space in the Storage Domain includes the capacity that is needed to accommodate the system overhead that is associated with the volume, such as the capacity that is required for RAID protection.
Insufficient capacity remains in the Storage Class or Storage Domain that you specified for the volume.
The system needed to round your request to a larger value, and the rounded value is greater than the available capacity.