These numbers take that into account - mouse over the device name on whattomine.com to see what mods/settings they're using for each device, f.ex the 570/580 use the Eth bios mod and 1100/2000 with a 0.2V undervolt. Pascal cards are undervolted as well. Without those mods you are going to use even more power and/or get a much lower hashrate (particularly on Vega).
Again: AMD is way, way behind on efficiency and has been since Maxwell. There are a few algos they do well on, but on the whole they are roughly a generation to a generation-and-a-half behind NVIDIA (Vega 56 is roughly as fast and efficient as a 980 Ti). You buy them because they're cheap for the hashrate, not because they're efficient. Think cheap-and-nasty here.
The thing with AMD power measurements that you have to be real careful of is that AMD's sensor is only reading the core power, not any power spent on memory or any losses in the VRMs, and they do it very inaccurately (and don't account for efficiency/etc). So you should be taking people's chatter on Reddit with a big grain of salt - unless you are measuring the power at the wall, or using a digital PSU, you are probably getting figures that are ~40W low, and potentially have 20% variation or so from the actual figure. Anyone using GPU-Z to measure their card is doing it wrong.
https://www.reddit.com/r/EtherMining/comments/674evm/rx_580_...