We have built into our games support for a range of resolutions covering most of the common resolutions of devices available at the time.
We build in some extra margins on these so that similar sized devices will still be covered.
Sometimes a new device comes out and its screen size (width x height in pixels) is quite a bit beyond what we've seen before. In this case the game tries its best to find a resolution it knows about in a similar shape, and scales the screen to fit.
Often when this happens there might be borders at the top and bottom (or left and right) of the screen as the game 'letterboxes' the view to fit.
If you're seeing this happen, and you have a device that you think might be reasonably common, please let us know as we'll want to accommodate it better for future versions!