The publishing system takes some pains to provide an accurate last-modified value for HTTP caching purposes (and an ETag too); internally it tracks the most recent modification time of all components that go into the page as it's (dynamically) assembled from various pieces.

(I am the author of the linked-to article and also the author of the software it's running. Said software also has (on-disk) caching, but that's not why the last-modified is back in December of last year.)