Usage in this case refers to how much of the disk that the directory is placed on is used. If you had a disk with nothing else but your directory it would show that you can still use the entirety (minus 1,2G) of that disk as storage, but that's not the case here. Space on the physically disk is taken up by the rest of the system. Thus, you can only store an additional ~80GB, not the ~980GB that the disk would provide if it was empty.
Hope that makes sense. You can see the used and available space on the disks that are available to your system with df -h
.