Hi again Gerald,
I think I see your misunderstanding.
When you open the dropdown menu "All" refers to all the courses on your Moodle site, not to all the all the exercises/quizzes in a QuizPort. On the Moodle site of my association "Grammar" is just one of several courses.
To decide which setting you want to apply (Site wide, Course wide or just QuizPort wide) you have to first set them in your chosen Default exercise/quiz.
I hope that the screenshot below makes it clearer that it is the setting of the Default Exercise 8 (which has no Pre-conditions or Post-conditions) which will be applied to to the "Grammar Course" (just one of several courses on the Moodle site) which contains many QuizPorts each in turn containing many Hot Potatoes exercises.
If, say, Exercise 2 had been selected as Default, all exercises in all QuizPorts on the Grammar Course would have the Pre-condition "Previous quiz" & the Post-condition "a score of 80% on Exercise 2. If applied to all the QuizPorts in the course it would cause disaster!
Applying default values course wide is a very powerful tool and should be tested locally before trying out on an active site. This is especially true if, like me, you have a large number of QuizPorts and exercises developed over many years with many different criteria for determining the optimum settings. Mistakes can wreak havoc. Yes, I speak from bitter experience.
But just removing the Pre- and post- conditions on a copy of your course shouldn't be too risky, though.