PayPal, which is one of Patreon's payment processors, was offering a deal there for a while – and while all this was going down in December of 2017, I got on the phone with PayPal and asked if the deal was still available and it was – where one could opt into an alternative fee structure, with only a 5¢ flat fee, but a 5% fee rate.
So one hypothesis is that that was what Patreon was using, either through PayPal or through another processor that offered equivalent terms.