Missing usable space on RAID5

I have 3 drives, 4TB each, in a RAID5 setup. Usable space is 7.24 TB, all good & checks out. I use 5.5Tb and it reports 600Gb left. Maths does not checkout. I get an external drive and I move 5.5Tb of storage off the RAID5 drives an onto a separate drive and the RAID5 drives report usable space of 7.24 TB. ~1TB of space was missing on the RAID5 drives and I don't understand why. I'm guessing something messed up with calculating free space, and moving files around forced things to recalculated remaining space? At the time... I tied using df and it reported free space as 600GB (so ~1TB missing) I tried du and it reported total used space as 5.5TB (so should be 1.74 TB remaining) I tried lsof | grep deleted and restarted any processes using deleted files, no difference Restarted several times Is this common on RAID5 setups? Is there something I can do differently if it happens again? Info on system in use... Device: TNAS F5-221 uname -r 4.19.165+ cat /etc/os-release NAME=Terra-Master Tnas VERSION=3.x ID=Tnas VERSION_ID=2018.04.9 PRETTY_NAME="Tnas 2018.04.9"