Minecraft wiki entry for Arrow, associated text: Arrows of Harming (and arrows of Healing when used against undead mobs) do not add a static amount of damage to the arrow. Instead, the arrow's damage is first calculated, then checked to see if it is below 12 × 6. If the arrow's damage is less than 12, the Harming effect of the arrow makes up the difference, to ensure the arrow does exactly 12 × 6. Therefore, an unenchanted bow cannot deal more than 12 damage using Harming (or Healing) arrows, as it can deal a maximum of 11 × 5.5 damage on level ground. However, if the arrow would deal more than 12 damage, the harming effect is entirely neutralized. This means that bows enchanted with Power I through Power III has a chance to not utilize the arrow at full charge, and any Power level above III never utilizes Arrows of Harming effectively at full charge when against unarmored mobs/players.