why are the back date files giving us 19.21 instead of 19.20 ?

From the firmware release history:

Changes made from version 19.20 to 19.21:

• Fixed issue with alarms intermittently not going off when the device is set in Max. Battery Mode.

I can't remember having any problems with 19.21 but I only used it between late September 2021 and mid-October 2021 when I updated to the 19.74 beta.

If you want a true 19.20 backdate file you will need to find a copy of the 19.21 Beta/Release Candidate archive for your watch.

This is the forum post for the 19.21 firmware:

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/273969/fenix-6-series---19-21-public-beta

(NOTE: The links in this post will take you to the current beta, not 19.21.)

Scillonian said:

If you want a true 19.20 backdate file

…have a look here : 

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/281427/i-need-the-backdate-file-for-19-20-not-19-21 

I don't need it myself but that will hopefully help others.