There's not really a limit on the space, the postid number just indicates that that is the true number for posts on GTPlanet, whether the ones that came before it were deleted or not.
It's because the number of posts is a simple count of the number of records in the database, but the PostID is a unique, auto-incrementing counter that gets set when you post.
So, for example.
Post 1 = Good
Post 2 = Good
Post 3 = Bad
Post 4 = Good
Post 5 = Good
Number of posts = 5.
Index of last post = 5.
If the poster of #3 realises his post is bad, and then deletes it, we are left with:
Post 1 = Good
Post 2 = Good
Post 4 = Good
Post 5 = Good
Number of posts = 4.
Index of last post = 5.
But the next post's ID auto-increments from the last post, the table does not fill the gaps, so
Post 6 = Good
