I think you should look in to the CSS for the Gridview for below one
<headerstyle cssclass="HeaderStyleGrid" />
It would be differ with the bound columns height
<asp:boundfield datafield="ItemCode" headertext="Item Code" headerstyle-width="80" headerstyle-height="20" itemstyle-wrap="true" xmlns:asp="#unknown" />
For bound column you have set 20 for height, but you might been set a different vale for the css class
HeaderStyleGrid, so check it. Also your question is confusing little bit so I gave this here based on my guess.