If you will notice, the signature in your first post change, also. That's because signatures are not stored with the post; they are stored with the members' account data. When a post is made the display engine attaches signatures to the post that qualify for them. (In all sub-forums, except this one, signatures will appear at most once per page.)
The result of all of that is that when you change a signature, you can look at any post that already contains a signature and see the change. Also, if you view your profile, you will find the signature at the bottom of the page.