Ferro potent armor gives DR when he starts regenerating health. So if someone damaged him before and he had DR while challenging you, it'd take more damage to kill him. Also depending on the class ability stat that also can give a small overshield on class ability use.

1) You could have dealt that damage over time, and his health could have recovered in the middle. Example: I frequently get thousands of points of damage but only 1 kill per round of Trials. Its because players heal up between engagements. 2) He could have an Overshield or some other healing item, like Wormhusk or Crest of Alpha Lupi. Maybe he was near a Rift. Considering you say he used x3 smokes on you, scenario 1 seems most likely. It takes a long time to recover x3 smokes, so that's a lot of opportunity for him to heal.