Maybe there's then something missing from the trade costs. I'll check this and will fix the manual if there's a mistake.
Update: No, trade costs are correct as far as I can see. The profit of a long Forex trade, with no swap and slippage, is:
(Endprice-Startprice-Spread)*TradeUnits - Commission*Lots*LotAmount/10000.
In case of a non-Forex trade with 1 lot == 1 contract it's:
(Endprice-Startprice-Spread)*TradeUnits - Commission*Lots.
This should produce the same trade results as calculated by Zorro.