There's also a bit of logic that goes into it. Not every day, every stand, every set up, every animal lens itself to a mer shot. That can only happen when things all line up good enough.
A guy got flamed hard a couple years ago for tweaking his mer on different days, but in the field that's exactly what I do. Certain conditions, like looking into the sun, being out of breath, strong breezes, super steep angles, very alert animals, etc all noticeably decrease my ability to hit behind the pin. It'd be foolish for me to think that because I've made the shot in practice under perfect conditions I'm automatically good to that range with any and every compounding variable stacked against me. The point of the challenge was to show what just one of those variables, a loss of shooting reps, does to your ability to make lethal hits.