There is a fundamental logic fault in the design of stats modules' queries.
The (original?) programmer presumed that each order will have an associated invoice but for many it's not the case (I am one of those many). The queries use 'invoice_date' where they should use 'date_add'.
The solution is simple: search and replace the first with the latter and number will appear. Use the quotes too, there is at least one function name containing invoice_date in one of the stats*.php files, you don't want to mess it up.