It seems to me that there's either an exponential factor to the damage a single attack can cause, making the final one or two health points much harder to take. It also seems that the first health point costs more to take. If you infiltrate an army, and take off that first point, you can take out a unit that would otherwise be left with more than just one point. What's for sure is that they've done something to make instakill very difficult. Even a Rogue against a Spearfighter will often fail to instakill, if the spearfighter has terrain bonus, and the Rogue's attack is 14+ times the Spearfighter's defense. That should be enough to leave nothing but a smear on the ground.
If anyone knows exactly what the attack/defense/damage equation is, I'd be very interested in hearing it.