When I was a kid, tuna and tuna fish were two different things.
Tuna was tuna straight out of the can, with nothing added. After all, the can says tuna right on it.
Tuna fish was tuna mixed with mayonnaise and crunchy things like pickles and celery. It was what today would be called tuna salad.
So if mom asked, "would you like a tuna sandwich for lunch?" we would reply "can we please have a tuna fish sandwich instead?" Because who wants plain tuna in a sandwich?
If she asked if we wanted a tuna salad sandwich, we would be completely confused. Is it a salad or is it a sandwich?