You really don't know how seniority works in a union house.
The job is broken down into many "bid positions". Each bid position consists of a very specific set of tasks. We have several for restroom detail, we have some for certain sections of the gaming floor and so on. Employees bid for positions in order of seniority twice per year. I bid for a position that does not include mops. My job is, lift, haul and dump. When you want any wiping, dusting, polishing or mopping done, call the person responsible for that particular bid position.
It also makes sense from a management standpoint because when a supervisor sees a thing that needs attention, they know who is responsible AND, they can more readily find out who is working and who is ignoring his work.