Why does normal redundancy for an x6-2 only allow 24T more?

Looking at the product specs for an X6-2 using High Capacity Disks and a quarter rack, it states:

    109T available for normal redundancy

       85T available for high redundancy

What confuses me, seems like NORMAL redundancy ought to be giving more space.

We also know:

    - there are 3 storage cells

     - each cell made up of 12x8TB disks (96TB raw)

In High Redundancy, the layout might look something like this

(C1,C2 & C3 are the 3 storage cells, "aa", "bb", are the file extents)

    C1     C2    C3

   ----   ----  ----

   |aa|   |aa|  |aa|

   |cc|   |cc|  |cc|

   |dd|   |dd|  |dd|

   |ff|   |ff|  |ff|