It seems like some sales reports do not take coupons into account.
Sales by category or Product sales seem to just multiply the number of sales with the price - no matter if some sales were reduced via coupon codes.
Sales by month do show the real sales amount.
Product "A", Category "Cat": regular price 10$
Coupon "X": reduction 50%
Two purchases: 1x full price, 1x with Coupon "X".
Sales by month report says: "Sales this Year: 15$" (correct)
Sales by category report says: "Month December 20$ / Total 20$ / Best Category: Cat 20$" (wrong)
Am I missing somthing or is this a bug?