SSRS absolutely sucks when it comes to clean exports to Excel. It loves to put in hidden columns and all kinds of other trash that makes the export useless to an analyst who intends to do further work with it. It's okay for just viewing the data but if you were only doing that, why would you bother exported to Excel? It's so bad, that for our analysts I am recommending we move their reports out of SSRS and make them natively in Excel and pull in the data through ODBC directly.