ZFS, like ext[34], also has a reserved space allocation for the entire pool to allow you to still do certain operations when it's reporting "0" free space (...l

ZFS, like ext[34], also has a reserved space allocation for the entire pool to allow you to still do certain operations when it's reporting "0" free space (...like deleting files or snapshots, to free space). Unlike ext[34], that reservation is not available for root's general use, but it's there.