GTX 1080 is lower hashrate than GTX 1070 on ETH, but somewhat better on everything else I've seen.
For Skein Groetsl and the ZEC algo in particular, it's about 15-20% higher hashrate than a GTX 1070 if they are set to similar "OC vs a Founder's Edition" settings.
Whattomine assigns hashrates arbitrarily, sometimes they are "reasonable" sometimes they are quite a bit off, and OC/UC/UV/TDP settings will affect that a LOT anyway.
Nothing they can do about that, given how wide a range of hashrates the SAME CARD (much less the same GPU in different cards) will demonstrate depending on settings and luck in the Silicon Lottery.
They do seem to assume "modded BIOS" cards on the AMD side, and some OC from most of the hashrates I've seen them use.
It's a useful site, but you need to know what YOUR cards will manage for it to be a good comparison site.
I just use the 1080 for ZEC. It is much better there.