Depends on the professor. I'll agree that most of mine have said "only answer what I've asked." In addition to that, some have added "I'll ignore anything extra" or, the more punitive version "I'll ignore anything extra unless it's wrong, and then I'll take points off."
I have screwed up on essays, such as when given the option to argue for either side, I was supposed to pick one or the other, and I picked both. In that case, I left both and just went on with the rest of the test.
But, in general, when they put in the call of the question, they want that information. If that's what you're providing, then it sounds like you're going about it the right way.