Whenever you make a post the new total is updated to all your previous postings, so that when you say things like "As a matter of fact, this reply is for 300!!" the associated posting totals will not match.
Whats the point? Do not refer to old posts by their number. The post number is meaningless as a identifier.
You can actually be anything you want! You do not need to be stuck with junior, senior, etc. Go to your user settings, and you can override the canned member blurb.
Another way to boost the number of posts you make is to add replies in which you just agree with what everyone's already said. It doesn't take much time, and you don't really have to do any thinking.