My uncle use to say that some times the hand of god reaches in and stops the bullet (or in you case arrow).
Who knows why you missed... only advise I have is to practice lots.
Shot placement is everything! I would rather take a round of 50BMG to the foot than a 22short to the base of the skull.
all 23 of my guns are 45/70 govt 357 mag, 22 or 12 ga... I believe in keeping it simple