First/last posts acting as if there is an ad on the right

[kellyryder]

In all of my first and last posts, the text is all pushed to one side and there's ad space, but I don't have any ads displayed nor do I want to display any ads there. Why am I getting this gap and how do I get rid of it?

The attachment is of a thread with only two posts so you can see what it's doing.

EDIT: I found one solution, which would be to set the stylevar for ad_post_maxWidth to 0 but I doubt that's the best solution.