If at the time you post you have no signature, the default setting is to not check the show signature box (makes sense - there is nothing to show). If you subsequently add a signature, it will only automatically appear for posts with the show signature box checked. You can manually change this post by post - not sure if that is possible in a locked thread (probably not).

Whereas, any posts with show signatures enabled gets updated with changes to your signature.

I'm guessing that when that thread was first started you had no signature, and subsequently added one. Looking at your final post on that thread, for example, there is a signature there.