Thanks for pointing this one out.
One thing to note in the T&Cs:
Das Aktionsangebot wird Ihnen nur bei Auswahl von Fernverkehrsbahnhöfen angezeigt.
In short, to see the offer, you need to select the long-distance station your train is leaving from in order to see the offer (usually the Hbf); if you select another station in a city as a starting point, the cheaper price will not appear in the results.