It's supposed to repeat horizontally, but there's no real way (that I know of) to stop it from repeating vertically without using a style attribute. Is that what you mean? If so you may need to toy with the font sizes inside the column headers; it might be spilling over due to browser font size settings.
Or did you already figure out how to fix it? I ask because I looked at your LJ profile and it looks just like the example on my end, but I have an outdated version of Firefox so it might not be able to see what's wrong (if there's still something wrong).