You have a SHR array using the two disks: in this configuration what you have is similar to RAID 1. The available space will be mirrored between the two disks and so the smaller disk size will dictate the available disk space. In your case 5.46TB.
The next thing to know is that advertised disk space (on boxes and other marketing guff) uses gigabytes and terrabytes using base 10. So 1TB = 1,000,000,000,000 bytes
But computers operate on a binary level and [since before marketing types thought it would be good to make their product seem bigger than their competion] your NAS and all other operating systems will report disk sizes in base 2. So to them 1TB = 1,099,511,627,776 bytes.
Now take your 6,000,000,000,000 byte disk and work out what the NAS thinks it is... 5.46TB