Vanguard S&P 500 question

[scottw]

Yesterday the S&P posted about a 6 point gain, but the Vanguard S&P Index Fund posted a slight loss for the day. Why did that happen?

 

[BASEHEAD]

Vanguard holdings are not kept in equal denominations.Different stocks make up a varied % of the funds total assets.S&P index is charted in a non weighted manner.