My guess is that you have rows of backlight LEDs along the edges of the display that lights up a plastic sheet from the sides. The emitted light pattern from each LED is spreading out at an angle. Take a paper, draw a rectangle 10x5 cm and mark every cm along the edges but not at the corners. From each marked location, draw a 3 cm long triangle that spreads 3 cm. You will see that most of the rectangle will have overlaps, but not at the edges or in the corners.
In order to save money they have not placed LEDs at the corners of the display. The technically best position would probably be at 45 degrees, which would differ from the rest of the LEDs and add cost.
On DSLRs you can find a similar problem, called vignetting, caused by a circular beam light that lights up a square sensor. The vignetting is more significant when the lens is stopped down (higher F-stop value, F11, F16...) which corresponds to a smaller diameter of the light beam that hits the sensor. The workaround is to apply a correction, as part of a lens correction profile, in the post processing.