I've looking at Drupal theming for a while now. I just had an idea yesterday though. The theme I was analyzing uses an H1 tag for the site title and H2 tags for the headings over navigation. I tried to decide if this made sense for me or not. On many sites this may be appropriate. It's possible that the name of the site is the most important thing and the navigation includes a heirarchy of decreasing semantic importance. It's also possible that the site is like many sites. The site title is a random five letter word and the navigation has a title like "Navigation".