That's the index value. It's really close if one factors in the dividends.

Do I get dividends???

Picking the Vanguard S&P 500 Index fund as an example:

https://finance.yahoo.com/quote/VFIAX/history?p=VFIAX
ADJUSTED CLOSING VALUES of Vanguard 500 Index Admiral (VFIAX)
(adjusted for dividends and other distributions)

1/3/22: 433.68 (date of S&P 500 index's all-time closing high)
12/11/23: 427.81 latest date (so down $5.87 (1.35%) from all-time closing high)
The last dividend payment shown in the above is 3/23/23.

Yahoo Finance is slow to post dividends.

According to Vanguard: https://investor.vanguard.com/investment-products/mutual-funds/profile/vfiax#distributions
There are dividends of $1.5781 per share on 6/29/23 and $1.4937 per share on 9/28/23
Simply adding those in ( $1.5781 + $1.4937 = $3.07) doesn't quite overcome the $5.87 deficit, but close.

I don't have a good source for UP-TO-DATE S&P 500 total return (including dividends and cap gain distributions). There are some sources that just give you the latest month-end situation, like Vanguard and Morningstar. Yahoo Finance is unique (AFAIK) in giving the info for any dates one chooses to calculate the total return between those two dates by using the adjusted close column. Except for one flaw -- they are often several months behind in posting distributions.

and make sure the math is correct and things add up. The statements show all the distributions and so on. (I admittedly rarely cross-check the distribution numbers with other sources and when I have I never found a discrepancy).

The problem is that they are month end statements, and so doesn't tell me, for example, how much the fund value is as of the latest close, and how it compares to that of a specific date like 1/3/22 -- the date of the all-time closing high.