[RedDevil]

Notice how the back window goes slightly steeper than the template, bending the airflow down at the risk of inducing instability... but it all lands on the spoiler which still slopes down, but not as steep. That creates a high pressure area over itself and the lower part of the glass, reducing drag.

In the ideal shape the angle reduction is limited to prevent instability, which would mess up the airflow over the tail. But the tail isn't there in practical road cars...

This is why the modern sedan shape beats the classical kammback. It uses the fact that it is hatched to full effect while the kamm shape is just a truncated ideal shape, and therefore no longer optimal.