You are wrong, every Nvidia Pascal Card, reduce the hashrate with every new DAG epoche. It's the same bug as the AMD R9 290x and R9 390 in the past.
The reduction is not much, but it will happened, You see it, when you let your miner benchmark in epoch 0 and for example in epoch 300. Test it and you will see it dude;)
My 1070's can easily reach 30-32mhs around 2018, current speed is 25-27mhs with the same OC as before, needed to increase core speed + voltage + "-straps" to reach 29-30mhs again.
Same with 1060's, from 24mhs, went down to 21-22mhs with the same 2018 OC.
RX's Polaris are not affected.