I was building a Ranger character with a Battleaxe in the main hand and a Short Sword in the off-hand... I had set the preferences to include the Short Sword as my off-hand weapon on the power cards. Given my Flashing Blade Mastery (+1 to weapon attack rolls with a light blade in the off-hand) and Attack Finesse (using my Dex modifier of +4 instead of my Str modifier), coupled with the +3 Proficiency bonus, that should make my off-hand attack with the Short Sword a +8 vs. AC. However, the Character Builder lists it on my Basic Melee Attack power card as a +7. I propose that this is because the Character Builder does not apply the Flashing Blade Mastery bonus of +1 to my off-hand attack with the Short Sword. It seems to be treating it as if it were in my main hand. The FBM wording makes it clear that the bonus should be applied to all weapon attack rolls as long as the light blade is in the off-hand. So, even if you attack with said off-hand weapon, the bonus should apply. It seems that the Character Builder either calculates an off-hand weapon attack as if it were a main hand weapon attack, or fails to apply the FBM bonus properly. Either way, this issue should be resolved in an update of some sort. Just trying to help!