confusing differences in total storage stats

[tarasnap@spag.fastmail.com]

Heya,

I've used tarsnap to back up a series of old cd/dvd/external drive 
backups, a lot of which were duplicate directories of photos.

It's costing more than I'd budgeted so I was looking to cut down what I 
was storing and wanted to see what was taking up the most space.

I used the `tarsnap --print-stats -f '*'` command from the docs but the 
output confuses me: the 'All archives (unique data)' line lists the 
compressed size as ~140GB, and this tallies with my monthly spend. But 
the sum of all the individual archives' compressed unique data only 
comes to ~41GB, which I wasn't expecting.

Things I checked:

- I compared the contents of `--list-archives` to `--print-stats` so I 
could confirm I hadn't somehow excluded archives from the stats.

- Possible cache-dir issues, but I can't see anything obvious:
    - Although the backups were on different drives I've run the 
tarsnap command from the same machine each time with the same key, root 
user, and default config (my /usr/local/tarsnap-cache dir is 948MB) so I 
don't think it's a cache-dir sync problem.
    - One of the backups did fail part way and I had to restart it (I'm 
UK-based and the process was *way* slower than I thought it was going to 
be!), but I used `tarsnap -d` to clear up the old and .part versions, so 
again I don't think that would have caused anything to get out of sync.


So either I've missed something or I've misunderstood how to interpret 
the stats, but I'm not sure how to figure out what I can prune if 70% of 
the storage is not directly attributed -- any pointers?


Thanks,

Tara