I believe that is because when the character doesn't have anything in their offhand, the menu considers them only wielding one weapon, so it doesn't show the possible change of the atk value of the offhand weapon.
Unfortunately, I don't have a good way to fix this, as the VS plugins are all obfuscated, and I don't have a way to hook into and change their menu scenes. It's frankly surprising that there was any change at all on that one.
Does it evaluate the proper way on the equip scene?
~Ramza