Heroes 6 - Discussion

I haven't been playing for very long, so I didn't quite catch one thing. Sometimes, my stack of melee units attacks only one stack, even though some damage is also dealt to the adjacent stack. But this doesn't always happen. Also, when playing as Necropolis, archers, dealing ranged damage to a specific enemy stack, also deal a small amount of damage to adjacent enemies. What causes this?

P.S. Another question arose. I read a fan-made manual and was surprised to learn that the Novice units of the Haven faction can heal, although only the Vestal (upgraded Vestal) can do this. Is this correct?