Projectile duplication

Projectile Duplication is a game mechanic present in the Genie Engine, although it did not appear in the original Age of Empires or its expansion.

When a unit or building with projectile duplication attacks, it releases multiple projectiles instead of one. However, only one of its projectiles will do damage according to the attacking unit's states. The rest of the projectiles will deal damage according to the projectile's own attack stats, which can be upgraded (or not upgraded) by relevant technologies. Because of this, discrepancies exist between displayed attack and actual damage dealt by projectile duplicating units or buildings.

Projectiles that do not hit its intended target deals half damage.

Chu Ko Nu (Age of Empires II)
Chu Ko Nu's projectiles have 3 pierce damage and 0 melee damage. It will do the normal 3 pierce damage on most units, but against the Battering Ram line it will do 3 melee damage, due to the ram line having -3 melee armor.

Town Center (Age of Empires II)
The Chemistry technology, which upgrades its projectile, will actually increase the Town Center's effective damage by 1 in the original CD version. It is not displayed in the HUD because it is not an upgrade on the Town Center itself.

Mangonel Line and Fire Ship Line
Because their projectiles have no attack listed at all, the extra projectiles are cosmetic. Only one will actually deal damage.