It's a great question. My thinking now is that he's waiting for k00b or davidw to leave comments (or the other top commenters) and he's zapping those right away. It probably takes many fewer sats to be the top early zapper of top comments than it does for zapping posts.

When we used to get the more detailed rewards breakdowns, I was almost always 2nd or 3rd in comment zapping. Maybe null was first (and doing it much more efficiently than I).