it's the way things work nowadays. A job is broken down into various parts which are then awarded to different subcontractors. Each sub contractor has a specific job to do and doesn't worry about where it fits into the scheme of things as long as they do their designated job and get paid for it.
I had my eyes open to something equally absurd in a new build school. One sub-contractor had the job of installing the door frames and another sub-contractor had the job of installing locks in the doors. The first contractor cut the holes to receive the mortice locks in the door frames. The other sub-contractor installed the locks in the doors at a different height to the holes cut in the frames.