Why it compares just one type of mobility from the very bottom to the very top? Shouldn't it change all the stages in between? Also, assuming we always have the

Why it compares just one type of mobility from the very bottom to the very top? Shouldn't it change all the stages in between? Also, assuming we always have the same bell curve of wealth distribution what does it matter who is where? Assuming some poor guy became rich, if the bell curve stays the same it means also that someone else dropped to become poor, so overall, what is the point? If you say the point is that having the opportunity encourage people to work and that in itself improves the situation for everyone then the American optimism is more valuable, even if it is not true.