As an example, the analysts looked at three different stocks, each of them showing the same annual average return over 24 months. Two were volatile, one was steady. The steady stock yielded a higher dollar gain than the two volatile stocks. That may sound wrong to you, but it's a mathematical truth. Volatility reduces growth.
Single stocks are more volatile than the market as a whole.
On average, they won't perform as well, over time. But naturally, you think you don't own an average stock. You own SuperCompany, one of the best. They went from brilliant to broke. There are many underperforming stocks, over the long run.