Why Are Aztec Bruce's Posts Deleted, Yet He Still Shows As The Last Post ...

It's probably just a "bug" that surfaced because of a decision made for performance reasons. The information shown on the board view (including the last post info) is probably stored in a separate table in the database from the individual posts. When a new post is made in a thread, the last post info for that thread is updated.. this way it can be read quickly for generating the board view page. The alternative would be to scan the posts table for the most recent post in each thread, which would probably kill the site's performance. A quick fix would be to make the script that deletes posts also revert the last post info to the previous post, but then you could run into timing issues where a new post is made just before an admin deletes a post, and then the last post info would be wrong anyway.

td;dr It's a minor bug that likely won't be fixed.