cross-platform backup tool Same files from different source dir causes spurious diff files

Greetings,

Recently I started experimenting with backing up LVM snapshots of live 
systems instead of the systems themselves. This protects against getting 
complaints about files changing like:

UpdateError: 
'clif/.mozilla/firefox/5zz41jpw.default/sessionstore-backups/recovery.baklz4' 
Updated mirror temp file 
'/leda.bak/home/clif/.mozilla/firefox/5zz41jpw.default/sessionstore-backups/rdiff-backup.tmp.75' 
does not match source

So for example instead of the source being the root dir of a remote 
server, it's now /mnt/vm-backup-snapshot/ on the local server. The 
destination is the same. The first time I tried this I got .diff and 
.diff.gz files for pretty much every file in the filesystem. The vast 
majority of these files are very small like 80-100 bytes. Here is a sample:


-rwxr-xr-x 1 root root 96 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/dir.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 103 Mar 18  2021 
tank-rdiff/rdiff-backup-data/increments/bin/systemctl.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 96 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/touch.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 80 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/rm.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 93 Jan 21  2019 
tank-rdiff/rdiff-backup-data/increments/bin/run-parts.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 91 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/chown.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 94 Mar 18  2021 
tank-rdiff/rdiff-backup-data/increments/bin/journalctl.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 83 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/sync.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 96 Mar 18  2021 
tank-rdiff/rdiff-backup-data/increments/bin/systemd-escape.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 109 Mar 18  2021 
tank-rdiff/rdiff-backup-data/increments/bin/systemd-tty-ask-password-agent.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 90 Feb 28  2019 
tank-rdiff/rdiff-backup-data/increments/bin/false.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 67 Jan  5  2019 
tank-rdiff/rdiff-backup-data/increments/bin/zforce.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 65 Jan  5  2019 
tank-rdiff/rdiff-backup-data/increments/bin/znew.2022-02-01T05:20:43-09:00.diff.gz
-rwxr-xr-x 1 root root 98 Jan 21  2019 
tank-rdiff/rdiff-backup-data/increments/bin/tempfile.2022-02-01T05:20:43-09:00.diff.gz

The total space used by this increment was around 500MB and the largest 
file was (the only file size that had a suffix):

-rw-r----- 1 root adm 2.1K Jan 22 05:21 
tank-rdiff/rdiff-backup-data/increments/var/log/auth.log.2022-02-01T05:20:43-09:00.diff.gz

I looked at some of these diff files and they seemed empty except for a 
binary header that I didn't find documentation for in the rdiff-backup 
file format section. I also noticed that occasionally, these "empty" 
diffs show up in normal day to day increments. I realize that diffs also 
can show up when the permissions change but I don't think all these 
files changed.

Is this how it's supposed to work, and could you please explain it to 
me? :-)

     Thanks,
     Clif