Because of the different sized screens on your laptop, mobile and/or tablet. So make sure the “main” part of your photo is centered in the middle. On a mobile screen your site shows in an upright format and on a laptop for example in a landscape format. That’s why it gets “cut off” in certain screens. To avoid this, choose the option “POSTER” instead of “FULLSCREEN” in BACKGROUND menu. The POSTER option fits your photo into the screen and downsizes it. Now depending on the screen you are looking at your site, you will either have bars to the left/right or bottom/top. In the menu you can choose white or black bars.