When adding storage to a system storage pool in Celerra, is there an advantage to using AVM (emc138143) as opposed to using the Celerra Disk Provisioning Wizard in Unisphere?

Also, with a 12 + 2 RAID 6 configuration with 2 TB drives, the Disk Provisioning Wizard shows it would add 17.91 TB of space.  Wouldn't you think it would add 24 TB?

Thanks...