Think it might be related to the category titles at the top of the page. This one has a long one ("Gatekeep's Post - Citizenship and Ambassador Applications"). v
This one has a short one and so fits correctly ("The Pride - LGBT Chat"). v
Not sure if this is related, but the main page doesn't exactly line up (you have to zoom out to see everything beyond the gold borders). There's no further empty space to the right, though. v
The same is also true for subforums (is that the right term?).