I think Stripe would be offering much better deals for high volume. With Stripe you are also paying for what they have built on top of just accepting the payment. If you are high volume and willing to use a no frills provider it doesn't surprise me that you would get a better deal.

We use PIN, a Stripe clone, for some currencies where they could offer us a better rate/ settlement. For our primary AUD transactions we are just using a bank on a much better rate.