That is because you are using an image for your logo that you want to stretch across the whole header, but when a user has a screensize larger than your image, he'she will see a blak space to the right.
That is why we include a blank logo in most of our theme packs to use,

what you need to change is you logo image.
Unless you want to completely remodify the header.html
You would need a background image to the right of the header in another table cell that can stretch if needed.