Okay because this is based on a pnp game you have to look at the dice side of things. for example a broadsword +2 would do 2d4 plus 2. So rolling two 4 sided dice for a result between 2-8 and then you add to for a range of 4-10. going through the probabilty curve is a diffrent story
. On a longsword +2 it would do a range of 1-8 plus 2 for 3-10. if its one of those weapons that are +1 and +3 vs undead . You add +1 to everything except undead which would be plus 3 to the roll.
hope this helps