Probably a combination of hold bonus, speed bonus, headshot bonus, and unlucky armor rolls.
If the stars align just so, you could do over 100 damage with an 8 PS +3 NCS head hit versus 58 head armor. The exact same hit (which includes a 20% speed bonus, a perfect hold, a head hit, no sweetspot penalty, and 100 effective WPF after armor reductions) could do as little as ~50 damage based on random factors alone.