Probably because you're using the default display option for this forum: "Show threads from the last 100 days."

If you go to the index page for this forum <---with this link, you should see (a.) three older threads and (b.) a Display Options device at the bottom for setting how far back you want to look. If you get to that page any other way, you may need to set the display options yourself before you'll be able to see those threads.