Why do dist-upgrade and upgrade give different held packages?

Question

sudo apt-get upgrade

gives:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  libavcodec54 libavformat54
0 to upgrade, 0 to newly install, 0 to remove and 2 not to upgrade.

and

sudo apt-get dist-upgrade

gives:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  libavformat54
0 to upgrade, 0 to newly install, 0 to remove and 1 not to upgrade.

Why when I do a normal upgrade are both libavcodec54 and libavformat54 held back, but when I do a dist-upgrade only libavformat54 is held?

Answer 1

Because they are different goals in terms of APT, apt dist-upgrade upgrades system by upgrading, installing and removing; apt-get upgrade upgrades by only upgrading packages, apt upgrade upgrades system by upgrading and installing packages.