Custom header – crop image dimensions

On the Ixion theme description, the Quick Specs section at the end of the page states, “The custom header appears at 1080 width with flexible height.”

I understood that meant the custom header image could be 1080px wide at any height.

However, when using a 1080px x 304px image, the Customizer restricts the image to about half that width in the “Crop Image” dialog.

Did I misunderstand what the size of the custom header image should be?