The Street.com looks at some of the implications of this difference:
"What is the impact of the different weighting schemes?....if one of the Dow stocks has a big move higher or lower, as 3M (MMM - commentary - Cramer's Take) did Friday..., the disparate impact on the indices is noticeable. The $7.29 drop in 3M accounted for 58.4 points of the Dow's loss of 134.63 points, or 43.38% of the index's move. On the same day, 3M accounted for 0.607 points of the S&P 500's 8.60 point loss, or 7.06%."The article goes on to talk about trading on the differences.
"The systematic impact of differential weighting over time should lead to a higher actual volatility for the Dow, and it does. Its standard deviation of returns since August 1974 has been 1.021725 times as great as that of the S&P 500....And even though the indices track each other remarkably well over long periods of time, considering their significant differences, they should not be viewed as substitutes for one another on a daily basis. If they were, we should expect both the regression coefficient and the r-squared to be far closer to one than the .855 and .7639 shown."