I had this problem, it's due to the database having sample data in it. Use phpMyadmin to edit your database directly and remove any items in si_invoiceitems that you have not created. Do the same in si_payments and si_invoices. Make sure you do a backup of your database first.
It will leave a few gaps in your invoice numbering, but your sales figures will add up correctly.
note: we have had a few issues in the past of similar reports saying different stuff - due to sql issues - but all is well now (last time i checked anyway :) )
