There are two sides that a closer can be mounted on, often called the "pull side" and the "push side". Which side it's on changes how the arm needs to be mounted. There are a few ways for that change to be made and the closer we're looking at is mounted in a way that .... "fell out of favour" I guess.

Instead of changing the way the arm attaches, there is a special mounting plate attached to the jam. When the door is open, the entire closer and mounting plate are out in the open space of the door. My best guess is that this obstruction that could knock a tall person in the head or get damaged by moving large objects through the opening (or just preventing smart people from trying) is why it's almost never done anymore.

HTH!