Du isn't accurate with btrfs, for single device cache the GUI is always accurate, and it checks out with btrfs stats:
Used: 78.92GiB
Free (estimated): 106.69GiB (min: 106.69GiB)
Free (statfs, df): 106.69GiB
These are is GiB, GUI shows GB, so they are the same and something is using that space, if you're not sure what is you can move some files at time to the array to find out.