I think the point is that if you want black coffee, there's no reason to say "cream" or "sugar" in the first place. "Coffee, one cream, one sugar" is a common p

I think the point is that if you want black coffee, there's no reason to say "cream" or "sugar" in the first place. "Coffee, one cream, one sugar" is a common phrase, and it's pretty to see how someone would think they heard that from the original phrase.