Everybody is above their own version of “average”
Everybody is unique. When people think of themselves in a positive light, they tend to think of the better parts of their personality, intellectual ability, physical capabilities, etc. Many people are willing to admit that they lack in certain areas, but they don't consider themselves normal; essentially, everybody is answering their own questions. If you asked an intellectual person if they were above average, they would most likely answer "yes," thinking in terms of their IQ and where it sits on the normal curve. If you asked a popular jock if he was above average, he would probably say "yes," thinking in terms of his fitness and popularity. If you asked an idiot that happened to be rich if he was above average, he would probably say "yes," thinking that money made him better. Nobody is on the same spectrum. They all have their own ideas of what average is, their own ideas of what makes someone successful, and almost nobody admits that they are above average in some random area but below average in other areas. Naturally, humans are pretty stuck-up and full of themselves; everybody thinks they are better than everybody else. So, if someone simply asked random people if they felt they were above average, most of the people would say "yes."