(I've shortened this post in hopes of making it clearer.)
The missing UIDs aren't related to previously-banned users at all. One of the missing IDs belonged to the BEA's owner, and I'm pretty sure he's never been banned. Something --probably a database-conversion problem-- deleted a small, random set of IDs, most likely during installation of new software.
All messages are considered public property once their posters' edit time-windows have closed, so when a user is banned, his or her posts remain accessible in the forum. Far as I know, no posts have ever been removed because their posters were banned.
Palomine agrees: this is my understanding too.
Now that this bug has apparently been fixed, if a post is missing and there's no posted explanation of the removal, then either the poster deleted it before his edit window closed, or a mod removed it and forgot to leave a note (which very rarely happens).