I can't say that this effect couldn't happen, Magnus effect or whatever, but how could anyone rule out the effect of wind eddies pushing the pellet up and down slightly? How could anyone be certain the airflow at that instant was totally horizontal? The air will be travelling slower at ground level than it is higher up. Even in a constant horizontal wind, this will create eddies where the faster air meets the slower air. If the ground was 100% flat and polished smooth this might be minimised, but eddies must be present on any outdoor range with grass or bushes or whatever.
I can't start worrying about a click up or down until I can consistently estimate how much left/right to give it, especially after last weekend!