Authorize.net not recording processor fees
Authorize.Net doesn't return this information, which is why it's not available in CiviCRM.  

One of the fields returned in the PayPal API documentation is transaction_fee. Authorize.Net's API documentation has no equivalent in the "Response" section.

The reason is that Authorize.Net can't tell you the transaction fee based on a single transaction, because their calculations are more complicated than PayPal's.  PayPal charges a high amount on every transaction, so they can give you a simple pricing structure.  Authorize.Net gives, e.g., variable rates based on volume, which can't be calculated until your month is over.