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The S&P 500 is a float-adjusted market cap weighted index. As of July 16 2021, the 2 top holdings are AAPL at 6.27% and MSFT at 5.78%, a ratio of 1.09. However the ratio of their market cap is about 2.4T/2.1T which is 1.14. How can this discrepancy be explained?

Both of these companies have 100% of float shares/outstanding

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The weights of S&P 500 constituents are rebalanced quarterly (every three months). In between the rebalances, the ratio of index constituent weights may differ from the ratio of their actual market capitalization. The index constituent weights are fixed until the next rebalancing, but stock prices (and market capitalizations) will always fluctuate.

Refer to S&P U.S. Indices, Equity Indices Policies & Practices, and Index Mathematics for details about the construction of S&P indices.

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