FredQ: I think the problem is your aspect ratios are different. To make an obvious example: suppose your layout is square at 400x400 with a background image the same size, and you export with scaling enabled to a phone with a display that is 800x600. Construct 2 will only resize the image proportionally up to 600x600, leaving 100px gaps to the sides. Depending on your settings (if you followed the 'clear background' mobile optimisations) you might get a weird glitchy appearance in the gaps too. So you have to design your background images to be large enough to cover all aspect ratios too.
It's the same problem as faced by TV and film: lots of shows have to be filmed in such a way as to look good on both 4:3 screens and 16:9 screens. This means avoiding things like actors or important events being shown right at the edge of the 16:9 view, since they will be cropped and invisible on 4:3. I think this is the same problem you face with your game.
Thunder what's going on?