What are the reasons for a parent to be harder on one of their children and not the other? They're also more overprotective but also abusive?

Physically abusive if the child makes a mistake. This child is quiet, emotional and never talks back. The other child is bold, talks back and wants to fight back so this parent doesn't really scold them as much. The quiet child isn't allowed to go out but the other sibling gets away with more

Whats the reason for this?
What are the reasons for a parent to be harder on one of their children and not the other? They're also more overprotective but also abusive?
Post Opinion