Atlas, that's been said two or three times already. (Unless the AP increased by 1 because the weapon switched ammo type from from slug to shot? In which case, that's not at all clear in the example.)
Kitsune, firstly, how do you get -2 AP? The example says there's only a difference of 1. Secondly, I doubt that was the rationale; it's more normal to have optional weapon qualities than it is for weapon values to be wrong by default.