I suggest you set Width="100%" for MasterTableView and remove ItemStyle-Width property from grid declaration and see if this helps to resolve the problem in Compatibility view. Keep in mind that you should use only HeaderStyle-Width for setting column widths.
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Setting the MasterTableView Width to 100% has worked great for my RadGrid controls that are using static headers in Internet Explorer 9, 10, and 11 Compatibility Mode.
This does not cause any issues with other browsers like modern versions of Chrome or FireFox either.
This is an awesome fix that has had me stumped for nearly 3 years. I had fought this problem and fought it in so many ways, using all of the browser tricks, forcing IE into different modes using meta tags, http headers, and developer tools.
Therefore, I have clearly re-worded this fix again in this post to further help the search engines.
This is going to help someone out there and make them very happy!