Почему dist-upgrade и upgrade предоставляют разные пакеты?

sudo apt-get upgrade

дает:

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.

и

sudo apt-get dist-upgrade

дает:

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.

Почему, когда я делаю нормальный upgrade оба libavcodec54 и libavformat54 сдерживаются, но когда я делаю dist-upgrade только libavformat54, удерживается?