DalekAddison - that's what the fullscreen scale mode does.
cesisco - the actual game viewport is not 960 x 640 - see the status bar at the top! That takes up some of the space so the actual game size is probably only something like 960 x 620. That has changed the aspect ratio so the problem is the same!
Since a lot of people run in to this, I've updated the supporting multiple screen sizes tutorial with this section:
b]A common gotcha - aspect ratios on mobile
Look carefully again at the above pictures - notice Construct 2 does not change the aspect ratio of the game. It zooms the window to fit, but if the view is too wide, gaps appear at the side. If the view is too tall, gaps appear at the top and bottom.
This means if you draw a background which exactly fits one aspect ratio, then run it on a phone with a slightly different aspect ratio, gaps will appear at the sides. Note that some mobiles and tablets will be displaying an address bar or status bar at the top of the window, which will change the aspect ratio again.
This is the same problem faced by TV producers. There are many TVs out there using aspect ratios of 4:3, 16:9 and 16:10. If a producer films a TV show only in 4:3, on a 16:9 TV there will be gaps at the sides. Similarly, if you draw a background exactly fitting one phone's window, then run it on another phone with a slightly different aspect ratio, gaps will appear at the side.
TV producers solve this problem the same way you should: everything is filmed in the widest aspect ratio 16:9. Then for other displays it's cropped down. For your games, basically you need to make sure you draw backgrounds a little wider (or taller, depending on orientation) than the window size, to ensure no gaps ever appear.