Not all weapons are equally effective against armored enemies. Eg. 'armor +25%'. This means that after the damage is rolled, and multiplied by appropriate Resistance value, armor value of the target is multiplied by 1.25, before being subtracted from damage. If it said '-25%', then target's armor would be multiplied by .75. Some damage types fully ignore armor by default (Charm, Burn, Choking, Hot, Cold). A weapon that both damages and reduces armor is dangerous, because the former occurs before the latter. Especially an area weapon against 2x2 units, as it inflicts 4 separate hits on them.