SPX index is also weighted (I think by companies market cap), which means some of the stocks have greater effect on the returns of the index. Also companies are

SPX index is also weighted (I think by companies market cap), which means some of the stocks have greater effect on the returns of the index. Also companies are periodically added/removed from the index as per their market cap which I think weeds out low performers without any bias that active fund managers/humans tend to have.