I'm not sure what your point is. If a function has a "unitary" return type (not, say, a list of possible return types out of which one is selected) then obviously it always returns a value of that type.

You could probably invent a kind of function that had a list of possible return types. Then anyone who called it would have to deal with those possibilities.

It all comes to the same thing in the end. Sum types and pattern matching are a very good system for expressing this functionality.

It's conceivable that a compiler would throw away the Either type information and just generate separate code for the different ways the function might return. Ultimately, though, I suppose it would end up calling a continuation, since the function has to "return" somehow.