Could be low powered ammo or that one round was short cycled during reloading, whatever. Just can't willy-nilly call REF and have them re-shoot because where do you then draw the line from within the calibration zone?
Rulebook shows you the calibration zone, clear as a bell. Even an edge hit falls into the calibration zone, but you don't call REF and give a re-shoot for an edge hit. Painting steel between shooters is a must, otherwise call REF on any edge hit and give any shooter another run at it.
I remember shooting the Buckeye Blast a good number of years ago, steel was set up at a good distance on one stage. My ammo had been calibrated at what I was shooting at that time (130 PF). I hit them dead center and one did not fall. Called for calibration. Bang, it fell. Just giving me REF would not have been justified or fair to the other shooters. That's when I moved up to 137-139 minimum PF for my rounds.