We just edited our site to clearly explain how the guest post box works.
The guest post box is shown only when your audience is looking at the first page of your blog through a browser. The guest post box does not show up when people look at other pages on your blog such as the second page or post specific archives. The guest post box does not show up in your RSS feed.
Furthermore, the post shown in the guest post box will change every time your audience visits. This ensures that there is always something fresh and new on your blog when your audience visits. Your audience expects to see new content when they visit.