Hi Deborah
It is designed this way so that the masthead (area with the site title/logo and menu) is overlaid over the top of the header background image.
This is because there is an opacity setting for the masthead background color, allowing you to set the transparency from 0 to 100 (0 allows the background image to show thru completely, 100 is the full block color).
If you want to adjust this, in the customizer go to ‘Extra Colors’ > ‘Masthead’ and for the Header background color selector there is a horizontal slider bar which can be adjusted from 0 to 100 in conjunction with the background color.