Shouldn't his health have at least dropped to 33?
I just checked, it seems it's as Stabbey said, Warfare seems to affect this value. Without any point in Warfare, the potion did exactly 30 points of damage (64 - 30 = 34). With 2 points in Warfare, which increases physical damage the character does by 10%, the damage became 33 (64 - 33 = 31).
What part of "drinking this potion deals damage to me" makes you think "I should drink a a second one"? What purpose did that serve?
He probably wanted to check the damage again, and by "safe" he meant with 35 HP left the potion shouldn't have killed him.
But hey you really should be careful with such things when playing Honor mode.