Acces rights to change Assignement (Responsible) to a Question

Hello,
I have begin to test acces rights and i have few questions about "Questions" Item :

For example that i want to do :

• User1 Create Question1 and assigne it to User2 by setting "Responsible" field.
• User2 can answer the question, can change status (base on rules workflow) but must not change responsible define by User1. (Fo me Responsible is the user who must work on this item, knowing that this item send to Todo page for this user, is it correct ?).

Can i do that i want ?

The test i did are as follows :
I have setted User2 Acces rights to "Creator" but i have notice that User2 is always assigned to Question1 but can't change anything on Question1.
So i wonder if this is normal ? Why Question1 can be assigned to User2 knowing that User2 can't change anything on Question1.

I think there is some logical i don't understand so can you explain or correct to me ?

Thanks you very much for your answer.