为什么 APT::Periodic 变量在两个文件中设置？

Question

...即在 /etc/apt/apt.conf.d/10periodic 和 /etc/apt/apt.conf.d/20auto-upgrades 中。

$ dpkg-query -S /etc/apt/apt.conf.d/10periodic
update-notifier-common: /etc/apt/apt.conf.d/10periodic

$ dpkg-query -S /etc/apt/apt.conf.d/20auto-upgrades
dpkg-query: no path found matching pattern /etc/apt/apt.conf.d/20auto-upgrades