All transactions in Auth.net are recorded as coming from the CiviCRM server
With most processors (including legacy Auth.net), if the Civi server is at IP address 1.2.3.4, and I make a donation from 5.6.7.8, the second IP address is what's displayed as the originating IP address.
However, with the new processor, all transactions are logged from the Civi server's IP address in Auth.net, which amongst other issues causes their velocity filter settings to not work as expected.