Greetings that don't make sense

You are walking down a hall, and you see someone you sort of know. He says "What's up?", and you answer "Sup." That didn't answer the question. A question begs an answer, unlike a greeting such as "hi", or "yo". But people are so lazy that they give a pre-canned greeting response, even when it makes no sense at all.

For instance, someone says "how's it going", and you respond "not much". That makes no sense at all, you were expecting him to ask what's up, but he didn't. And you were too lazy to alter your response.

I don't really have a point, other than it's awkward and annoying. Pay attention to you greetings.