Still not following. What do you mean by "monthly mean precipitation"? Do you mean the monthly total precipitation?
If so, what do you mean by "agree with my calculations"? How did you calculate February's total? Directly from the database? Or, are you saying that the total given for February on the page NOAA-2020.txt does not match the total given on NOAA-2020-02.txt? Or, something else?
It would help if you included the NOAA-2020.txt and NOAA-2020-02.txt pages. Or, even better, if they are online, include a link.
There just isn't enough information to diagnose the problem.
-tk