At the time the newsletter is written, they pull the most current posting for each challenge, unfortunately, by the time the newsletter is emailed, either the challenge has been up for a few days or a new one has already been posted and the link in the newsletter is "old". It is just one of those publishing hazards
If you want to stay on top of the challenges, check out the listing on the bottom-right of the home page - it tells you what challenge is on what day (and there is a challenge every day of the week!).
HTH!