apt instale a versão “mais antiga” do pacote

Question

Ubuntu 16.04, docker, nenhum pacote instalado. Eu tenho o pacote A , que depende do ffmpeg:

# apt show A
...
Depends: ..., ffmpeg (= 3.4-7), ...
...

Eu tenho uma versão mais recente (do que um sistema) ffmpeg no meu aptly repo personalizado, mas como 7: é menor:

# apt policy ffmpeg
ffmpeg:
  Installed: (none)
  Candidate: 7:2.8.11-0ubuntu0.16.04.1
  Version table:
     7:2.8.11-0ubuntu0.16.04.1 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 Packages
     7:2.8.6-1ubuntu2 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
     3.4-7 500
        500 http://packages.tnt/aptly/public/orion/ubuntu xenial/master amd64 Packages

Quando tento instalar o A :

# apt install A
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 A : Depends: ffmpeg (= 3.4-7) but 7:2.8.11-0ubuntu0.16.04.1 is to be installed
E: Unable to correct problems, you have held broken packages.

Mas

apt install ffmpeg=3.4-7 && apt install A

funciona!

• Por que apt tem esse comportamento?
• Como fazer com que apt install A funcione como esperado?