Commission and Prices

The total price which my guest has payed is 155,40€.

What is the commission free price? In this case ist is 137,40€.

Then there is also still the commission and the Bank fee, which are 20,61€ and 1,74€.

My payout ist 137,40€ minus 20,61€ minus 1,74€ equals 115,05€.

Why does it start counting minus from 137,40€ and Not from 155,40€?