Unfortunately I've never worked on a true stepdown or PCU. They all had it in their name but we never had the 1:3 ratio. 4 on dayshift was the goal but 5 happened alot. 4 was the goal on nights as well which almost never happened. Usually it was 5 with occasionally having 6....all the while being expected to manage the drips, post-caths with arterial lines while also being on a nitro drip. Both floors did not take vents.
It sounds like your hospital is using that reasoning as a way out of having to pay more staff. It happens at alot of hospitals unless that hospital has a strong union. Most "stepdowns" or "PCUs" are glorified M/S tele floors.