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.
Actually ETH runs better with a lower core clock speed compared to ZEC. It also must be something about the difference in memory architecture. My MSI 1080 Gaming X will do ~560 H/s on ZEC, but only ~25 MH/s on ETH. The difference is the 1080's use GDDR5X, while the 1070's use GDDR5 like the RX 580. The 1070's also run at a higher core clock compared to the RX 580's.