Why is the size of the widget not the same as the size of the screen?

Hi! I use NGUI for my GUI. I have UIWidget, which is stretched and anchored to full screen, as shown on screenshot:.

It is said that its size is in pixels. So why it is not the same as screen size, which is also in pixels? If I log to the console this code: Debug.Log(widget.width + ", " + widget.height); Debug.Log(Screen.width + ", " + Screen.height); // Screen resolution. It gives different results: 591, 1280 1125, 2436 Is it because UIRoot? How is it works?