In our experience, the post office oftentimes lists a package as "delivered" when it really hasn't been. It can take up to 24 hours after it's posted as "delivered" for it to actually be delivered. While we can only speculate as to why this happens, it does happen occasionally.
If you don't have your package and the post office says it has been delivered, please wait an additional 24 hours for delivery before contacting the post office. Of course if you suspect theft or prefer to inquire immediately, please contact the post office for further details.