Border surrounded my iPhone screen

About a year and a half ago, I bought an iPhone5s.

I have used it during the past without any problems at all. Until one of my friend saw a very dim border surrounded my iPhone screen.

It was very dim, so it can not be detected in normal conditions. I can see it only when the color of the screen is white or a light color.

The easiest way to be able to see that is put the machine is tilted at an angle of 45 degrees when startup or when opening the application "Notes". Please refer to the attached picture of my iPhone. I want to ask about the cause of this phenomenon? Is this the same state with other new iPhones5s when leaving the factory or not?

1. Start screen

2. Notes screen

3. Normal screen