I guess there's an algorithm that stops pks of the same person or people that you kill over and over. When I'm randomly pking I'll notice after killing any one person three times in a row it stops counting them as a pk until I kill a few different people.
I also noticed pk streaks are not counting up either. I had to have killed at least 100+ people these past couple days without dying and I still only have a streak of 57. I'd have well over 100 of a streak right now. Pity.