The background image will never fit perfectly in the browser unless the browser window is the exact same ratio as the image.
The default settings on the SeedProd plugin use the Cover property for the background-size. This is defined as: Scale the background image to be as large as possible so that the background area is completely covered by the background image. Some parts of the background image may not be in view within the background positioning area.
Basically what this means is that when the images do not fit it gets cropped. You can set the focal point in the plugin using the background-position property.
You can use the Contain image setting to force the image to stay contained within the browser and parts that don’t fit will show the background color.