Why might a scenario-based estimate be more accurate for a short-run expected return estimate than a historical

Question:

Why might a scenario-based estimate be more accurate for a short-run estimate than a historical AM estimate?
Expected Return
The expected return is the profit or loss an investor anticipates on an investment that has known or anticipated rates of return (RoR). It is calculated by multiplying potential outcomes by the chances of them occurring and then totaling these...