lets see. if we want a three and we have a 2, we need one to make 3. If we dont have one we have everything else we need to make three but the one. Therefore the +1 is responsible for the 2 becoming a 3 (success) compared to the 2 without the one (fail).
The two still was a two on both sides, but the lack of the +1 given already having a two means the lack of +1 is responsible for not having a 3 (success)
Basic stuff here. Im not sure why you don't get it. You used a black and white term, i did not. Your black and white term is wrong.