Hi,
I’m experiencing a difference in what I see in the Customizer http://prntscr.com/obn6nf and what I actually see in the header of the site http://prntscr.com/obn6vf
Why is that happening and how can I fix it?
you have a 100% wide image on a full width header. Thee image resizes to fit the browser viewport.
So the difference you see is because the customizers viewport is not as wide as the front end due to the setting panel. If you reduce your browser width to match the viewport of the customizer when viewing the frontend you will see the backgrounds look the same.