Dreamwidth style system discussion

I sometimes post a line (code is <hr>) in my entries to seperate topics and I noticed that the line doesn't spread out over the entire lenght of my entry holder, like it should and does in some of my friend's posts. I don't know if that's due to some code in my CSS or if there's another reason for this. However, I would like to change it. Does anybody know how?

P.S. I don't have an example image, but I can make one if necessary. Thanks in advance!