Three possible reasons, in order of frequency:
- When someone is banned with “Remove Content,” only the ban shows up in the modlog, not the post/comment removals. I’m not sure if this is a bug or intended behavior.
- Sometimes a modlog entry fails to federate to your instance. This sometimes happens with posts & comments as well. Federation is “best effort,” and can fail for a variety of reasons.
- Occasionally, especially egregious content, like CSA material, is “purged” rather than removed. No record of the post/comment remains. I think these still show up in the modlog as “a post/comment was purged” (or something similar), but without any of the content, because it was too offensive even for the modlog.