[Agema]

I think the reflection of AP on weapons like axes, maces and so on is not that they penetrate the armour per se, but that armour could much less effective at stopping the damage from them compared to a sword. Armour was generally about stopping penetrative damage from thrusts (eg. spears, swords) and slashing (swords). Weapons like maces and warhammers were designed to apply a lot of force into a very concentrated area. Even if the armour wasn't penetrated (in the case of plate armour probably leaving a huge dent), the blow could pulverise bone and flesh under it.