Why would a package be installed from a repo and not another?

I use systemd (xenial) and want o be up to date. To do so, I install Martin Pitt's PPA and everything is fine.

On one system (not special in any way, this is a clean install actually), despite the PPA being configured ...

root@eu11:~# apt update
Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:2 http://fr.archive.ubuntu.com/ubuntu xenial InRelease
Hit:3 http://fr.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:4 http://fr.archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:5 http://ppa.launchpad.net/pitti/systemd/ubuntu xenial InRelease
Fetched 102 kB in 0s (297 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.

... systemd is installed from the standard repository.

root@eu11:~# apt show systemd
Package: systemd
Version: 229-4ubuntu17
Priority: required
Section: admin
Origin: Ubuntu

What can be the reason for such behaviour? (I did not configure any pinning because I do not know how to do it, never having had the need before)

This is the output of apt-cache policy systemd :

root@eu11:~# apt-cache policy systemd
systemd:
  Installed: 229-4ubuntu17
  Candidate: 229-4ubuntu17
  Version table:
 *** 229-4ubuntu17 500
        500 http://fr.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     229-4ubuntu10 500
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
     229-4ubuntu4 500
        500 http://fr.archive.ubuntu.com/ubuntu xenial/main amd64 Packages