Differance b/w the way of hiding nav bar in iOS swift

Recently i had one issue. When in my navigation controller stack (One child controller) :

MainScreen -> ScreenA -> ScreenB -> Screenc

So when in my Screenc i want to hide nav bar and status bar.Its was working fine. But when i come back to my ScreenB my status bar and nav bar are overlap.Not sure why its happend.I search in some google and some SO answers. Then i got that isNavigationBarHidden base is UIViewController while isHidden base is UIView.

Why i hide my both nav bar and status bar with below code :

 override var prefersStatusBarHidden: Bool {
        return true
    }
 override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
       self.navigationController?.navigationBar.isHidden = true
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
       self.navigationController?.navigationBar.isHidden = false
    }

In my prev screen i got the issues of my status bar and nav bar are got overlapeed.

But when i use this code :

 override var prefersStatusBarHidden: Bool {
            return true
        }
override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        navigationController?.isNavigationBarHidden = true
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        navigationController?.isNavigationBarHidden = false
    }

Its work well. There is not overlap in my prev screen. Why its happening. Is there any specific reason behid :

navigationController?.isNavigationBarHidden ( vs )self.navigationController?.navigationBar.isHidden

I am very to the iOS Development. Just want to understand the difference. So that i can learn if i am doing any wrong.

Thanks