Some of that is going to be playstyle. It should also be noted that it lasting 12 turns doesn't necessarily mean much as a battle contract might of involved shooting for what 10 (sometimes 11) of those turns? Machineguns are support weapons and are generally considered to be a good weapon to cause a head hit. SRMs are also pretty good at causing headshots as I recall for various reasons. What's your approach to the mission? What units were the enemy? And then of course the RNG could of just hated you that mission as well.
From the description you mentioned being knocked over 1 time and 1 torso destroyed but you didn't note if the pilot you lost was hit from a called shot on the head while on the ground or by machineguns while prone. The pilot would of been more vulnerable in general. As for the 2 head hits you scored how did you get them? From just random fire, precision shot or did you knock a mech down?
It was 12 combat rounds, I don't count the 'close the gap' turns. Also, I had the high ground. I'd understand it more if I was on a lower elevation.