JC's stylesheets specify widths for the columns being displayed. He has seperated the categories so that each is a table of it's own in the display. The unmodified forum index displays as one single table and therefore the columns always line up.
This isn't a problem with the stylesheets offered here. It's just that these stylesheets here were not created with JC's templates in mind.
(and honestly I don't think they should be made with them in mind because not everyone is using JC's templates)
Edit Note: After looking at some of JC's stylesheets it seems he doesn't always specify the widths in the columns in question. I had ran into just one such case a while back on someone else's site. Pulled my hair out for a while wondering why the columns were not shrinking... lol
