Usable Space of Euallogic RAID50

Hi,

i got a new PS6100 with 24 x 1TB NL-SAS HDD.
The raid policy is RAID50 with 2 spares.

can someone explain why the usable space is limited to 15,71 TB ?

I thought that there are 2 striped RAID5 (n-1) sets on that machine if i chose RAID50.

I start with 24 x 1 TB = 24 TB

- 2 spares = 22 TB

- 2 drives parity data (1 for each RAID5) = 20 TB HDDs for DATA

Each TB HDD has about 930 GB usable space.

930 GB x 20 = 18600 GB

18 TB is not equal to 15,71 TB

 

What is wrong with my calculation?