Some nocks are slightly different width and can a have an effect on nocking point levels, but if a nock with a snap on design has any amount of tightness that will affect how the arrow comes off the string. I find that even if those clip on nocks just kind of grab they can come off the string differently from shot to shot. I have one in my practice set, sometimes it goes with the group and sometimes it wanders around in flight. I break lots of nocks in practice, I usually shoot at that arrow, but that nock is the luckiest nock in the world. I cannot seem to hit it, one of these days I am just going to cut it off.