non-Euclidean filesystem

I noticed that I only had 5 GiB of free space left today. After quickly deleting some cached files, I tried to figure out what was causing this, but a lot was missing. Every tool gives a different amount of remaining storage space. System Monitor says I’m using 892.2 GiB/2.8 TiB (I don’t even have 2.8 TiB of storage...