In Iron Man 3 Stark first meets this woman (Brandt) who was an Extremis soldier in a bar. He was able to kill her with an explosion.
However, in an earlier scene, another Extremis soldier (Savin) who was also caught up in another explosion, survived. Furthermore, he was also shot in the head by Stark after he killed that Brandt woman but he, again, survived.
Brandt's body was still intact after the explosion whereas Savin suffered more serious injuries (he has a hole in the head after being shot by Stark, for example). How come he survived while the other one didn't?