The discount is calculated on the price calculated in the booking form. For 30 nights it is 79.500, so if monthly discount for example is set to 20%, then the price will be 63.600 for 30 nights.
The Airbnb "average price" better be ignored, it is often not correct. Also Airbnb considers 28 nights as a "month". Airbnb only looks at daily rates, if you have set fixed prices for specific months it will also fail in the "average price" calculation.