When on green, why does S40 default to par level score?

I switched from TomTom Golfer 2 watch about 2 months ago to Garmin S40. I still find the interface of the watch less intuitive than the TomTom. What I am constantly puzzled about is when I enter the green - the watch prompts to ask how many strokes I played. I know it doesn't track puts - but how come it always defaults the number of strokes to the green to the level par number? It has just tracked the number of strokes I played to enter the green - why isn't that number not the starting value (after which I can then enter the number of puts).... ?