The non-magnetic is a better grade of stainless steel with a higher chromium and nickel content than the lower grade tank's metal. I always took a magnet with me when I went shopping for a new tank back in those days. (They don't call me old man for nothing) Mild steel, the kind that rusts easily, was magnetic. The better grades of stainless steel were not.
Steel is not just iron. Good quality stainless never rusts and is non-magnetic, at least on the simple levels that we saw in the aquarium business.