apt install "older" version of package

Question

Ubuntu 16.04, docker, no packages installed at all. I have package A, which depends on ffmpeg:

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

I have newer (than system one) ffmpeg version in my custom aptly repo, but since 7: prefix it is lower:

# 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

When I try to install 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.

But

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

works!

• Why apt have such behavior?
• How to make apt install A work as expected?