From what I see, I'd bet the author is not taking into account dividends and other things. What you should look at is the "Total Return" c.f. https://tradethats

From what I see, I'd bet the author is not taking into account dividends and other things. What you should look at is the "Total Return" c.f. https://tradethatswing.com/average-historical-stock-market-returns-for-sp-500-5-year-up-to-150-year-averages/ https://tradethatswing.com/average-historical-stock-market-r...