It's only 1 shot=1 kill if it's a headshot. Anywhere else it's at least 2 shots.
I kind of like it this way, so unless more people agree with you, I won't change it. But thanks for the feedback, maybe others will step in and agree.
You can change the damage yourself if you like by editing interventiondamage.gsc
Terminators have 500 HP.
so this part:
says that all damage to helmet, head and neck (considered headshot area's) will do 500 damage, which is all the HP terminators have and thus resulting in a 1 shot kill.
If you want it to take two headshots, change 500 to 250.