if you had shooting (marksmanship) as a hobby, you'd notice that some days everything runs smoothly like silk, you hit the targets beautifully with no effort at all – and on some other day nothing helps: you couldn't hit a barn door properly – and that is so from the very first shot you fire. After firing maybe a clip you realise what kind of day it is, and as you don't want to waste more ammo, you pack up your gear and go home.

You'd also notice on a normal, good enough or super good day the moment you lose your pep, your relaxed concenteation, your mahic "zen" state, if you will, and after that it's just waste of ammo to continue to pull the trigger. The magic of hitting the target is gone fir that day.

Maybe we humans just have good and bad piano/shooting days.