The gap above the scroll is a legacy from the first versions of RadControls for ASP.NET AJAX. We prefer not to change this appearance as we will have to introduce breaking changes which is not acceptable.
However, you can workaround it by making a new image exactly like a header and set it as background of RadGrid with CSS. You can find a sample page in the attached file. (sample-grid_CSS_workaround.zip)
Also, you can workaround it by filling the gap with new elements, which are created with the following javascript and styled appropriately
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