How to use `tmutil compare snapshot_path`?
From man tmutil: 

snapshot
  A directory inside a machine directory that represents a single initial or
  incremental backup of one computer. The word "snapshot", in most contexts, is a
  generic term and is not to be confused with a "local Time Machine snapshot",
  which is simply a snapshot stored locally on the computer. E.g.,

    /Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456


I read this as "a local snapshot is not the same as a snapshot", with tmutil compare only able to handle the second.