S&P 500 Historical Annual Performance vs. Dow

After today's selloff, the S&P 500 is up 7.8% for the year (ex-dividends) while the Dow is only up 2.9%.  This means that the S&P 500 is outperforming the Dow by 490 bps, which seems like a lot given that the indexes are both large cap indices.

Still if the indexes ended the year with this performance, it wouldn't be the largest historical spread between the two.  In 55 years of S&P 500 history, there have been 10 years that it has beaten the Dow by more than 5% (ex-dividends).  There are also 9 years that the Dow has beaten the S&P 500 by the same spread.

Makes you think--what's the point of benchmarking active managers if even similar benchmarks outperform one another from year to year?