The diagram is confusing because DB has started “massaging” the data. As an example, if a train which travels back and forth between two cities has a certain amount of delay, it will just turn around and not serve the remaining stations on the line. And that’s as many stations not counted as delays. This trick is named the “pofalla turnaround”, after the DB infrastructure boss Ronald Pofalla who introduced it in 2018.

And I think that’s just one trick, meaning the downfall starts earlier and is more significant I suspect.