And the contents of the delta:
$ tar --wildcards -tvf /srv/system-image.ubuntu.com/www/full/pool/ubuntu-631ce387d6c729c3da0ae9177d0c9d9a07f3e0dfb15b21b20771079322915d20.delta-ubuntu-fa74c536de07b74b0c7484c4e20ebc471a5965bc784b1a38face0b8e7598316a.tar.xz system/usr/bin/python3.4'*' -rwxr-xr-x root/root 2887488 2015-03-27 02:51 system/usr/bin/python3.4m $
So system/usr/bin/python3.4m is removed and then re-added - leaving system/usr/bin/python3.4 unmodified (and thus stale).
And the contents of the delta:
$ tar --wildcards -tvf /srv/system- image.ubuntu. com/www/ full/pool/ ubuntu- 631ce387d6c729c 3da0ae9177d0c9d 9a07f3e0dfb15b2 1b2077107932291 5d20.delta- ubuntu- fa74c536de07b74 b0c7484c4e20ebc 471a5965bc784b1 a38face0b8e7598 316a.tar. xz system/ usr/bin/ python3. 4'*' usr/bin/ python3. 4m
-rwxr-xr-x root/root 2887488 2015-03-27 02:51 system/
$
So system/ usr/bin/ python3. 4m is removed and then re-added - leaving system/ usr/bin/ python3. 4 unmodified (and thus stale).