Help understanding annual inflation vs average inflation, and which to use for converting nominal returns to real returns

[https://www.usinflationcalculator.com/inflation/current-inflation-rates/](https://www.usinflationcalculator.com/inflation/current-inflation-rates/) There is a table on this website that shows the US monthly historical inflation rates, and then the average for the year. Above the table it says that the rate listed in December represents the annual inflation rate for the calendar year, while the rate listed in the average column shows "the average inflation rate for each year using CPI data." I'm not really understanding the distinction between these two rates. Ultimately I'm trying to figure out which of those two rates to use in order to convert my nominal investment returns to a real return %. Can someone help me understand which rate I should be using?