The 'clusters' that you refer to, Agchristie, probably come about simply through some 'tidying up' by a moderator.
On the few occasions that I've deleted an individual post there have often been a string of comments about that post immediately after it. Deleting the original post alone would leave loads of posts which would simply baffle people joining the thread, so it's best to remove the lot.
For example, an intoxicated member might post a string of 4-letter words. The next half a dozen posts simply say 'Reported', 'Disgusting', etc but they'll all become meaningless when the original offending post is zapped, so the whole lot need to disappear if the thread is to retain any integrity.