We are using Facebook Audience Network in random order with other ad network for mobile ads. FB ads earnings are about 3-5% of other networks, with exactly the

We are using Facebook Audience Network in random order with other ad network for mobile ads. FB ads earnings are about 3-5% of other networks, with exactly the same traffic. Other networks disclose the percentage the revenue split between them and the publishers, but FB does not. What do they overestimate in this case, I wonder?