2K screen resolution problem

I have a function for adapting UI in different resolution:

for the width:

public float Vx(float x) { float xPos; xPos = (Screen.width * x) / 1920; return xPos; }

and for the height:

public float Vy(float y) { float yPos; yPos = (Screen.height * y) / 1080; return yPos; }

When i set the position or the size of a UI element, i write Vx(400) instead of 400, and it works.

But if i create a sprite with the resolution of the whole screen and i use resolutions like 2650 x 1440 (galaxy s6 edge) or 480 x 320, the sprite doesn't cover the entire screen. why? there is a better way to fix the size/position in different resolutions?