RX 580 vs GTX 1060 looks rather ugly also. A few patches have no doubt been applied since - anyone know if this this situation has changed?
Likely comes down to ACC's use of DX11 and Unreal Engine 4. Nvidia has better optimised DX11 drivers, and i've seen a number of reports that nvidia has a closer relationship than AMD with UE4 developers.
It's in nvidia's commerical interest to influence developers to favour their hardware, and judging by what we have seen from nvidia re: gameworks i wouldn't put it past them putting in effort to bring about the situation we see above.
Regardless, AMD customers of course will want to see AMD offer an explanation for such situations and action to address.