The document discusses how average stock market returns of 10% per year over 80 years obscure the actual variability in annual returns, which ranged from -43% to 56%. It notes that very few annual returns were actually between 9-11% and that planning based on average returns does not account for the reality that returns can widely vary. The document suggests that starting points in the market that differ can have significant impacts on outcomes for investors due to the non-normal nature of returns.