Its because it is true. Growth does not imply fast growth. Does a tree have explosive growth it's entire life? The material that is growing I believe is cartilage, and it grows in those areas. Have you ever looked at a photo of your grandparents when they were young and compare them to when they were old? (Sorry if grandparents are not alive) The growth takes place slowly over a long period of time with the change only being noticed after a comparison.