What do you mean, "to armor"? This is not damage done to the armor value of the target, but extra armor applied before damage is dealt to the target.
Damage on target works like:
( WeaponPower * [0-200%] ) - Armor
If a weapon has +30% armor, it does:
( WeaponPower * [0-200%] ) - Armor*130%
So a bigger number is subtracted from the damage dealt.
Similarly, a spiked mace with -30% armor, would do:
( WeaponPower * [0-200%] ) - Armor*70%
So a smaller number is subtracted. Maces deal more damage than other weapons against heavily armored targets, shovels (and many cutting weapons) deal less.
Shovels do mainly cutting damage, extra stun from weapons is handled entirely differently. Each time you apply damage to an enemy unit, a small amount of stun damage is dealt. The stun bonus applies to that, so a shovel will do twice as much of the little bit of stun damage.