First boss task... got a question

Hey so yesterday I got my first boss task.... Kree'arra. I'm in a fairly large clan so I went with the max # of kills (35). I'm only 81 ranged but I went in a trio with two other clan mates with 99 ranged. We got like 20-25 kills that trip.

Now here's my question: I got two actual Kree'arra kills. Like, I got the drop on two kills. However, a total of 8 kills counted toward my boss task. So 6 kills when I didn't get the kill.

How is that determined? I didn't get the kill 6 of my 8 "boss kills" but they still counted. Is there any rhyme or reason?

Thanks!!