I think it has to do with memory clocks on the cards. AMD RX 580 clocks top out at ~2100 MHz, while Nvidia clocks run at more than 5000 MHz.
so ETH requires more the core clock which is close between the cards, ZEC uses both (core + memory) and there Nvidia comes in advantage because of high memory clock.
wondering if some miners could be optimized for core that AMD cards can raise their S/s.