Try when you have 501.

If that still fails, I can edit for you as well- so can any other mod.

two reasons

Firstly, The difference between your member profile and the figure next to your post is something to do when the forum was upgraded, and how topics are deleted. Sometimes they dont minus from the first, But do from the second figure. Also someone (*cough* Mur) pressed a button they shouldnt and "recounted all posts" During the upgrade before last, meaning that everyones post are out of sync with the two figures. The lower being the amount of posts minus any deleted or "lost" posts.

The second reason as shadow pointed out, is that im not sure when the system "checks" if you can edit it. Each time you post, it checks if you have the correct amount to be able to be "promoted" to another group. Now if it checks before incrementing your post count, then you will need to post 501, instead of 500. Others have experianced the issue where they can only actually get the title once they have X01 posts instead of being dead on the hundred.