Because your background image size is – “contain” and it won’t cover all the layer area that you set at slider settings http://prntscr.com/ra7kxp
If you want to avoid that space then use bg without text and choose bg size “cover” http://prntscr.com/ra7m49 and add text using text layers. Using “contain” you will always get space either at the right/left side or top/bottom depending on the device width.
