Your BTRFS partition (device /dev/nvme0n1p6) has several subvolumes (var, home and root), identified in your first screenshot with their corresponding mount points (/var, /var/home and /sysroot).
The second screenshot, however, presenting the results of the Disk Usage Analyser, only shows the details of the Home directory (i.e. user’s Home folder within the home subvolume).
You’ll need to check what takes up the rest of the space (e.g. other users’ home directories, if any, OSTree deployments and containers in /sysroot, Flatpaks and their runtimes in /var/lib/flatpak, BTRFS snapshots, if there were any taken etc).
It is also documented that the df command can be unreliable when run on BTRFS file systems. Please also observe the specific features of atomic desktops, where the root file system has lots of symbolic links and mount points, many of the system folders being created at run time from the booted RPM-OSTree image deployment.
