For one thing, QDirStat explicitly tells you ">23.1 TB", i.e. MORE THAN 23.1 TB because it could not read all directories below /data/appdata, probably because of insufficient permissions. The part of the screenshot that was cut off (below the tree) also notified you of that.
But even then there will probably be a mismatch because of ZFS snapshots which, like Btrfs snapshots, also are not reported by ZFS to the relevant system calls like stat(). You will need dedicated ZFS tools to give you that information.
HTH
HuHa
QDirStat author