Deleting face only possible if angled above 90° with adjacent?

[Sagespahne543]

Hello forum:

I'm coming from the Autodesk self-paced learning video series "Direct modeling",
where faces are deleted in different contexts.

In my example,  identical profiles are extruded, each only differing in one angle.

I am deleting the longer, angled face (on the left).

The faces of the front of the body are not on the same height, so I am expecting Fusion to extend the angled face instead of extending the front one. 

However, the angle between the selected and opposite face has to be above 90°.  Why is that?

Thank you for any answer

[CGBenner]

@Sagespahne543 

Hi, have you solved this or do you still need some help with this question?