I know this is a bit old, but I was running into the same problem. The issue is, the report is deducting the sales tax from the product sales, which it should not.

In admin/includes/classes/sales_report.php line 385, I changed it from

$product_price_inc_tax = (zen_add_tax($final_price, $tax) * $quantity) + zen_add_tax($onetime_charges, $tax);

to

$product_price_exc_tax = ($product_price_inc_tax - $product_tax);

Now it calculates correctly. It's a quick hack, but it works. Someone could take the time to make a more elegant solution. I don't understand why it's doing this since Sales Tax is clearly additive to the total.