This is just the UI size. On the Roku devices, you have a graphics plane (which is either 720p or 1080p) and a video plane. The video plane is the native output size for the display and video is automatically scaled for it. The graphics plane is where non-video content is displayed.
Currently, 4K devices support both 720p and 1080p UI, while HD devices only support 720p. This is usually due to the amount of memory available on the device, since FHD graphics require more memory to display. Also, many devices have limited graphics acceleration capability, which makes putting a fluid UI on screen at higher resolution problematic.