Because `net_amount` is implemented in [children](https://github.com/neighborly/neighborly-balanced-bankaccount/blob/im-transaction-fee-calculators/app/models/neighborly/balanced/bankaccount/transaction_additional_fee_calculator.rb#L7-L9) [classes](https://github.com/neighborly/neighborly-balanced-bankaccount/blob/im-transaction-fee-calculators/app/models/neighborly/balanced/bankaccount/transaction_inclusive_fee_calculator.rb#L9-L14).