Sub Tasks

There is something quite weird happening:

-I create a Main Group: aMasterTask = aProject.AddTask()
-I create a sub group: aTask = aMasterTask.AddTask
- I fill this sub group with tasks
-I create a sub group: aTask = aMasterTask.AddTask
- I fill this sub group with tasks

The problem is the following:
I should have

-MasterGroup
- Sub Group 1
- Task 1
- Task 2
- Sub Group 2
-Task 3

But instead I get:
-MasterGroup
- Sub Group 1
- Task 1
- Task 2
- Sub Group 2
-Task 3

aProject.AutoOutlineLevel = True

solved the problem