Perhaps because the ammo for this gun has some bad Armor Class modifiers.. they acually add to the critter's armor class and thus lowering your chance to hit, as an offset they give huge bonus to lower physical damage resistance, which helps a lot with armord targets, and damage in general.
On the contrary JHP ammo or jacketed hollowed point ammo or shotgun buckshot shells have negative armor class modiiers, bu positive damage resistance modifiers, which reflects their true physical properties, it's easier to hit with this but a lot less damage to armored targets. they also have a general damage multipiler which is sepose to simulate more damage done to unarmored targets, which they both do in reality.