It's those pesky
15 pixels of padding:
This layout is 600 pixels wide. A banner image automatically has a 15 pixel border in a basic template, this is the green area that you see around the banner. The color of the border is set on the Design > Style page for the template, in the Page Banner element with the "Background" color.
To create a banner that fits perfect, you would subtract the 15 pixels for each side border (30 pixels total) from the width of the layout, creating a banner that is 570 pixels wide. The height will adjust automatically.
Here's a message from tech support about it:
Currently the basic template has an automatic 15 pixels of padding around it, to create a frame. There's an example in the above link.
We're planning on adding the option to adjust this in the basic sets, but to remove it at this time would take an advanced template.
More info here.
