I'll hazard a guess it was a bad shot, aimed upwards,
'Upwards' could describe quite a few trajectories. A shot straight up will lose it's kinetic energy and tumble on the way back down. But a shot fired with some elevation (say 45 degrees) will still follow a ballistic trajectory past its maximum altitude. So it is possible that this round was fired in such a manner and encountered the LE slat from above while still flying pointy end first with significant kinetic energy.