Question about the boltshot

When you fire a charged shot, it drains 5 bullets out of the clip, meaning that you can fire 2 charged shots per clip. However, if you drain 9 out of the 10 bullets (by semiautomatic fire), and use a charged shot for the one last bullet in the clip, it will still one hit kill and do the same damage as any other charged shot. What kind of logic is that?