When somebody SPAMs a thread, they will show as last post, even if their post is removed, until some scheduled refresh cleans up the list of last posts. In other words, the forum CACHES the "last post" information, probably in a separate DB table, and then has to go and clean up the table when posts are removed.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.