To reproduce:
 - Add a table where the width is set to 100%
 - Import the table and the export it:
private void radButton_Click(object sender, RoutedEventArgs e)
{
    var provider = new HtmlFormatProvider();
    string text = File.ReadAllText(@"..\..\default.html");
    rtb1.Document = provider.Import(text);
}

private void radButton1_Click(object sender, RoutedEventArgs e)
{
    var provider = new HtmlFormatProvider();
    provider.ExportSettings.StylesExportMode = StylesExportMode.Inline;
    string content = provider.Export(rtb1.Document);
    File.WriteAllText(@"D:\test1.html", content);
}

Workaround:
 Leave the export mode intact.