Comparing the Shuck Shot to the standard ZPG you'll notice that he ZPG is one projectile while the Shuck Shot consists of two projectiles fired side by side.
This is an important distinction because when you miss with either it's equally a miss but they aren't necessarily equal when they score a hit. Because the ZPG is a single projectile a hit inflicts the full damage but the Shuck Shot has its full damage divided up into two projectiles and both have to impact the target to score the full damage and that comes into play because a hit with the standard ZPG will tend to vanquish your target but because the Shuck Shot might hit for partial damage you won't necessarily vanquish your Zombie opponent.
