Dual backstab

Your backstab MASSACRES a mist wraith! A mist wraith alertly twists out of the way of your second backstab. A mist wraith is covered with bleeding wounds. Is there some nuance to dual backstab that isn't listed in the helpfile? Literally, the wraith twisted out of my dual backstab 100 times in a row when I landed my first backstab. I've landed some dual backstabs while ranking, but when I go to practice on low level stuff, it seems like they always twist out. But the wraiths did it over 100 times in a row.