At the time of writing, <https://launchpad.net/ubuntu/+archivemirrors> reports that the Shanghai Linux User Group mirror is "One week behind". But the mirror's individual page, <https://launchpad.net/ubuntu/+mirror/mirrors.shlug.org-archive>, reports every single variant as "Unknown freshness". One of these must be wrong.
This happens because in the +archivemirrors page we use DistributionMirror.getOverallFreshness(), which gets the worst status different than UNKNOWN, unlike the getSummarizedMirroredSourceSerieses() (which doesn't ignore UNKNOWN) used in the mirror's home page. That's why we sometimes see this discrepancy in the status reported in the two pages.
See also bug 70855.
Adding a new Ubuntu series will cause that series to be unknown in all mirrors, so this happens at least every 6 months to all mirrors.