How does S20 know record where we putted from?

Had the same question!!  How does it know your approach shot?

https://forums.garmin.com/outdoor-recreation/golf/f/approach-s20/266645/there-s-a-flaw-in-calculating-approach-shots

I guess nobody wants to tell. 

Last week I particularly wanted to know exactly how far I hit a 6 iron onto the green because it went further than I thought.  On approaching the green by walking around to the back  (about 180m+ by now),   and selected "Measure Shot" on the watch,  and it had no measurement,   i then selected Previous swing and it advised 166m... how did it decide where it finished i havent hit another shot yet - i could have hit it 20m long?        

Then on completion of my round I see in the Garmin Golf app,  the data reports that this 6 iron to missed the green right,  which also means its now incorrectly recorded that statistic,  and I cant even correct that.      

it now seems Garmin decides at random where the approach shot finished.

You hit it on the head.  I guess when you tell the watch you hit a 6 iron, you need to be right where the ball came to rest.  That might be the answer to my original question.