Как делает механизм нейтрализации способной работы обновления в случае проблем соединения?

При установке обновлений сегодня, я заметил это:

Get:15 http://in.archive.ubuntu.com/ubuntu/ trusty-updates/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2 [626 kB]
Err http://in.archive.ubuntu.com/ubuntu/ trusty-updates/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2
  Connection failed [IP: 91.189.92.200 80]
Get:16 http://security.ubuntu.com/ubuntu/ trusty-security/main libdns100 i386 1:9.9.5.dfsg-3ubuntu0.2 [626 kB]

Была некоторая проблема соединения в загрузке пакета от моего зеркала регулярного обновления, http://in.archive.ubuntu.com/ubuntu/ и updater выбрали http://security.ubuntu.com/ubuntu/ в качестве нейтрализации для загрузки того же пакета и обновления, за которым следуют после этого. Это - первый раз, когда я заметил такой механизм нейтрализации при загрузке обновлений.

Однако следующее является содержанием моего /etc/apt/sources.list файл:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

# newer versions of the distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty universe
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates universe
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://in.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu trusty partner
# deb-src http://archive.canonical.com/ubuntu trusty partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main

Поскольку каждый видит, только trusty-security репозиторий включен для security.ubuntu.com. Однако пакет, который имел проблемы, был от trusty-updates. Так, как этот механизм нейтрализации работает? Кроме того, каковы сценарии, когда этот механизм инициирован?