HDMI screen zoom - not full 1080p?

I bought a UG007 android hdmi stick to watch films in 1080p on my hdtv. however, when I start the device, it doesn't fill the whole screen. It starts to fill it only once I turn the slider in the "screen zoom" part of the settings. I worry that I may not be getting full 1080p image but rather something upscaled. With the screen zoom slider at its lowest setting, it looks like this:


The problem (?) is not solved even if I put some other firmware (Finless Rom with a 1080p kernel) on the device.