backup_dir problem

[Steve Marx]

I am having a problem using the backup_dir option 
of rsync.  The command being used is basically

rsync --archive --delete --backup
   --backup_dir=PathToBackup/save
   user at fromsystem:FilePath/ PathToBackup/

The feature that is unique to this problem is that the
"save" directory is under the path where the files are
being backed up.  Fortunately, rsync is clever enough
to not delete the PathToBackup/save tree even though
there is no corresponding "save" tree on the "from" side.

Now here is the problem.  On the first run with
modified files, the old versions are  saved in
PathToBackup/save. However, on the second run,
those files are moved to PathToBackup/save/save
and the "new" old versions are placed in 
PathToBackup/save.  Each subsequent run 
produces another layer of the "save" tree.

This is not the behavior i am hoping for. I
would just like a single matching directory
tree with the most recent old-version of each
backed-up file that has been modified.

So is this a bug?  Am I doing something wrong?
Does this behavior make sense somehow? 
Thanks for any help or suggestions.

 Steve