I don’t know why I didn’t think about/realise this before. When knowledgepool displays a ‘related posts’ search, the search is in realtime – i.e. it searches at the time the page is displayed, NOT at the time the post is made.
Of course, this means that any related posts (remember those decent keywords, kids) posted in the future will show up in the related posts of archived posts – see thisold post on css. What a brilliant, unintentional side-effect.