Your results are different than what I saw here, https://whattomine.com/coins
For example, vega 56 does 15 mh/s, in your case 20.5, i wonder where those 25% come in.
Vega 64 17mh/s, in your case 23 mh/s, so different than what I saw on whattomine.
WTM also states 30Mhs on eth for rx 580 but in reality it does 31.5+
You mus be trolling right? I do not know much about progpow but for ethash i know a lot and I can say that 30 is maximum average modded gpu. For example, some gpus will do 28.5 and some like you said 31.5 and so 30 is the medium number.