I can't open your post for editing on that forum, but I think the problem might be where you put the [ARI] text all the way over on the right. hr by default won't stretch a page, but if a page element causes a stretch, I think hr will fill in as much of the stretch as its able to.
Edit: Ninja'd :3
Seahawk -> RE: Fixing lines (11/2/2012 9:29:14)
Heh... I CAN open your post for editing (Admin powers FTW!) and yes, it is definitely the [ARI] that's causing the issue. Since you're using [code][/code] tags in that section to display your code, it's also forcing the forum to maintain the spacing in front of the [ARI] and pushing it to the right side of the page. The HR is merely extending itself to fill the width of the post; it's not actually stretching the post. If you cut out some of the spacing before the [ARI] you'll drop back to normal post width and won't have an issue.