My only guess is that unless they realized belatedly that the person had Supervisory skills/experience in that area. And felt like they would be a better person to replace someone else with that was leaving. Then I have no clue how that would work out either.
We have a lot of cross trained supervisors, sometimes we have had 5 or 6 working at the same time. Only one is in charge though, along with Front End Lead (in charge of all of FE) if FEL is working, with the others either doing Service Desk or cashiering, though normally whoever is CSR on duty will also monitor Uscan as well. Down by me we simply go with the flow, if we get told something, we do it. Doesn't matter to us, as honestly we are all there to do a job. I've been told three different things once by both CSR, Store Director, and a night Uscan attendant, so I just go with the flow.