Why is apt-get ignoring the target argument?

Question

My Debian system is set to install unstable packages by default

$ cat /etc/apt/apt.conf.d/99defaultrelease 
APT::Default-Release "unstable";

However, I would like to pull automake from stable, ie, version 1.15

$ apt-cache policy automake
automake:
  Installed: (none)
  Candidate: 1:1.16.1-4
  Version table:
     1:1.16.1-4 990
        500 http://deb.debian.org/debian testing/main amd64 Packages
        500 http://deb.debian.org/debian testing/main i386 Packages
        990 http://deb.debian.org/debian unstable/main amd64 Packages
        990 http://deb.debian.org/debian unstable/main i386 Packages
        990 http://deb.debian.org/debian sid/main amd64 Packages
        990 http://deb.debian.org/debian sid/main i386 Packages
     1:1.15-6 500
        500 http://deb.debian.org/debian stable/main amd64 Packages
        500 http://deb.debian.org/debian stable/main i386 Packages

But whenever I tell it to install automake/stable or automake with -t stable, apt just ignores that part.

$ apt-get install automake -t stable --dry-run
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  autoconf autotools-dev
Suggested packages:
  autoconf-archive gnu-standards autoconf-doc libtool
The following NEW packages will be installed:
  autoconf automake autotools-dev
0 upgraded, 3 newly installed, 0 to remove and 125 not upgraded.
Inst autoconf (2.69-11 Debian:testing, Debian:unstable [all])
Inst autotools-dev (20180224.1 Debian:testing, Debian:unstable [all])
Inst automake (1:1.16.1-4 Debian:testing, Debian:unstable [all])
Conf autoconf (2.69-11 Debian:testing, Debian:unstable [all])
Conf autotools-dev (20180224.1 Debian:testing, Debian:unstable [all])
Conf automake (1:1.16.1-4 Debian:testing, Debian:unstable [all])

I can specify the package version like this:

apt-get install automake=1:1.15-6

Which should solve my problem, although the dependencies (autoconf, autotools-dev, etc.) will still be installed from unstable, which is a little concerning.

Anyway why is apt ignoring the given target in this case?

Also - this is not a question - if this behavior is "by design", I think it would be really nice if apt could print some informative notice.

Edit

This is the output of apt -t stable policy automake (note: after having installed automake)

$ apt -t stable policy automake
automake:
  Installed: 1:1.16.1-4
  Candidate: 1:1.16.1-4
  Version table:
 *** 1:1.16.1-4 1001
        500 http://deb.debian.org/debian testing/main amd64 Packages
        500 http://deb.debian.org/debian testing/main i386 Packages
       1001 http://deb.debian.org/debian unstable/main amd64 Packages
       1001 http://deb.debian.org/debian unstable/main i386 Packages
       1001 http://deb.debian.org/debian sid/main amd64 Packages
       1001 http://deb.debian.org/debian sid/main i386 Packages
        100 /var/lib/dpkg/status
     1:1.15-6 990
        990 http://deb.debian.org/debian stable/main amd64 Packages
        990 http://deb.debian.org/debian stable/main i386 Packages

Since this might be an issue with priorities, this might also be relevant:

$ cat /etc/apt/preferences
Package: *
Pin: release a=unstable
Pin-Priority: 1001

Answer 1

The behaviour you’re seeing comes from the priorities you’ve defined. Setting the default release, using Default-Release or -t, sets the priority to 990 for corresponding versions; your priority of 1001 for unstable always beats that.

See man apt_preferences for details.