I am guessing that Bob looked around for other posts of his and found they all had the same number. Therefore, he concluded that his number wasn't changing, while in fact it always changes. The number seen on a post is not the post number you have made when you made that post, but the number of posts you have made overall. It's just a bit of global information that is stored and displayed on your posts just like your name. If you change your name (as Dave Schmidt is wont to do), it too will change on all your posts, and all your PM, etc.