Sometimes, I check my Xbox Live achievements for how many AvA kills I have when I enter, and how many more I've scored when I'm done (mostly when trying out new builds).
If I run solo, and just "zerg surf", go literally solo, and jump between keeps and kill enemies on resources, I can easily have like +300 kills in an evening. If I stick to a group consistently, I can have like 60 kills during the same amount of time. Sure, I run with a group playing objective (accused of "taking empty keeps") that doesn't seek combat, but occasionally we clash with big well known groups, sometimes we win sometimes not. But, I don't have a feeling I kill so few enemy players, but numbers say different.
Same thing with the conquest thing Kill 40 Enemy Players. If I go solo, I have that one killed in like 20 minutes. If I'm grouped it can take hours. I'm like "Yes, THAT kill was mine! Huh? No apparently not...".
What really determines if a kill is counted as yours? I have even seen obvious glitches with the 40 player one; facing like 2 guys completely alone - killing them both without any allies even being near, and your count towards 40 isn't changed at all. Does it have to be unique players (i.e. killing same guy several times doesn't count).