Cultures vary over space and over time. What was true of rhetoric for the ancient Greeks and Romans is not necessarily true for us. We can apply much of their work directly to our own rhetorical practices, but we also have to modify much of it to make it fit. Consider what some of our Asian students must learn. Whereas Western cultures tend to prize originality and individuality in student writing, Asian cultures tend to prize preservation of cultural values.
Asian writing is more likely to spiral on itself, reiterating ideas in various ways.
Many students who come to us from foreign countries have to learn not only to speak and write in English, but also to follow the rhetorical conventions of a foreign culture. We may speak of cultural difference in other ways, too. Imagine you enter a parlor. You come late. When you arrive, others have long preceded you, and they are engaged in a heated discussion, a discussion too heated for them to pause and tell you exactly what it is about.