I don't know that comments are deleted so much as some go into a spot where others review them before they are posted.
It may be some, or all, and may depend on whether commenters are "learned" to need review based on ratings, or history.
It seems an interesting algorithm, likely the crux of "Civil Comments" merchandise, i.e. what The Herald pays for with their service.
I've had one deleted, but, since I don't remember what I wrote I don't know why it didn't make it.
And, I've seen when you don't log in (having cleared cookies) a comment is not there that is there when you log in. Later, it will be there, hence why I think it goes through a period of review before being displayed for all.