Dead Island: Mods change the Stats of weapons?

Question

When modding weapons, I often see numbers beside particular stats of the weapon I'm modding. For example, when I want to create a Nail'd Baseball Bat, there will be a (+1) beside the weapon's damage in the modding menu. These numbers and stats change depending on the weapon used. Modding a Baseball Bat with Torch shows (+1) to damage, but this doesn't happen with a Heavy Baton. How much do these bracketed numbers increase? Is the damage increased by 1 only, and if not, does anyone know the exact increase in the stat?