Why are the edges trimmed?

Regardless of its physical size, TV hardware manufacturers reserve space around the displayable area of the screen. This reserved space is known as overscan. The amount of space a TV uses as overscan varies across manufacturers. Make sure you design your content in a way that no text is placed in this area.

Although Android and Amazon Fire TV platforms provides a way for the user to adjust for television overscan in the settings,  we recommend that you avoid placing important content within the outer 5% of any edge on the screen. All of your messages should be fully within the inner 90% (the safe zone) of the images.