When I look at monthly S&P 500 prices at a site such as Multpl, they state "all prices are monthly average closing prices." e.g. https://www.multpl.com/s-p-500-historical-prices/table/by-month
How does one calculate this "monthly average closing price?"
For example, they report 3,668.10 for December 1, 2020. When I average all the closing prices for December 2020 from https://finance.yahoo.com/quote/%5EGSPC/history/, I don't get that number. When I average all the closing prices for November 2020, I don't get that number.
I see other sites citing 3,668.10 for December 2020, so it looks like there is a canonical method for calculating it.
Is "monthly average closing price" the simple average of all closing prices in the month? If so, why doesn't the reported average match when I calculate it?