Ready NAS 3312: Storage Space is not Showing correct

RAID-6 requires 4 drives of the largest size in order to utilize all the space on the disks.  You only have two 8 TB drives installed, so the RAID subsystem is treating this as 6x4TB (ignoring 4 TB in each of the two 8 TB drives).

6x4TB RAID-6 yields 16 TB of storage, since 8 TB is needed for dual redundancy.  The NAS reports storage in TiB (1024*1024*1024*1024) not TB (1000*1000*1000*1000).    If you convert 16 TB to TiB, you get ~14.55 TiB.

If you were to add two more 8 TB drives, you'd end up with 32TB of storage with RAID-6.

Switching to RAID-5 (single redundancy) would yield 24 TB of storage.

Unfortunately for you (mostly because you didn't know in advance), XRAID automatically switches from RAID5 to RAID6 on a rack-mount unit once there are 6 or more drives.  It does that on the 7th drive on 8-bay desktop models (so 6-bay units don't do it).  Had you switched to FlexRAID, you could have forced RAID5, but you'd be forever stuck in FlexRAID.

You can manually switch the RAID6 to RAID5 with this command via SSH:  

mdadm --grow /dev/md127 --level=5 --raid-devices=6 --force --verbose

It will do a RAID resync and then report the change in RAID type and size when it's completed.

But that will still leave 4TB of each of the 8TB drives unused.  In addition, each added drive will again switch it to RAID6.

You can manually create an MDADM RAID2 from the two unused 4TB segments and then add it to the BTRFS volume (which it what XRAID would do to use that space).  This post may help: How-to-do-incremental-vertical-expansion-in-FlexRAID-mode.  Note that I have since determined that you don't actually need to be in FlexRAID mode, but you may want go go there so future expansions don't cause XRAID to do things you don't want it to.  The downside of doing that is there is no return to XRAIUD once the volume is vertically expanded and that all expansion is now manual (though some can be done visa the GUI instead of SSH).