Assume you select your portfolio by selecting darts at the list of all stocks trade in the United States. Shockingly, you outperform the S&P 500 index. This happened because:

You are a particularly good at darts.

You mostly pick small stocks, as they are more numerous on your list, and the small stocks outperform.

The market is random, so a random strategy outperforms.

You pick stocks that do not go bankrupt.