You must configure the device information settings to use simple headers:
rsreportserver.config. Mine C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\
excel .
<Extension Name="EXCEL" Type="Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer,Microsoft.ReportingServices.ExcelRendering" Visible="False">
<Configuration>
<DeviceInfo>
<SimplePageHeaders>True</SimplePageHeaders>
</DeviceInfo>
</Configuration>
</Extension>