Let's say you've got a ping-pong ball and a huge inflatable Swiss ball like the ones you find in the gym."How different are they?" means "TO WHAT DEGREE are they different?" or "BY WHAT AMOUNT are they different?". You'd answer, well, they're VERY different because one is tiny and the other one is huge. How different are they? A lot! But how different are a tennis ball and a green billiard ball? Not VERY different because they're kind of similar sizes and they're both green. They're not very different. Now imagine a red billiard ball and a green billiard ball. "How are they different?" means "what property of the objects has different values?". The answer "They're different colors". That's HOW they're different. Now imagine two red billiard balls. How are they different? They're not; there is no property of them that differs between the two. How different are they? Not at all.