A 60/40 portfolio looks balanced on a pie chart. In risk terms it's closer to 80/20 — the stocks are driving almost everything, and the bonds are decoration.
Take the classic 60/40 portfolio: 60% stocks, 40% bonds. Now put rough volatility numbers on it — stocks around 15% a year, investment-grade bonds around 5%. The stock sleeve contributes 0.60 × 15 = 9 units of risk. The bond sleeve contributes 0.40 × 5 = 2. That's 82% of the portfolio's risk sitting in one asset.
— Educational, not financial advice.