Probably because in real life, if you get a sharp thing stabbed through your forehead it's automatic Death. However, this thing does not happen to Terrific Spongebob Darts and Minion Darts as they do not really have a forehead. As for Acool Beauty Darts and PSY Darts, they are humans so it's possible to one hit kill them.
I always hate that white duck in the Glacier where in order to beat him, you have to have another duck that can't fly at all. What a waste of time grinding.
 "Crocodile Circus Show"){kind=icon}
 "Justin Bieber Darts"){kind=icon}
 "DuckLife 4"){kind=icon}