People tend to that when it suits them, they're both on the maybe could K.O 100k tier, not Shanks could and Ray maybe or maybe not.
What your brain don't understand is that fishmen are all weak fodders and Fishmen Island Luffy was simply too weak to finish all, thus only KOed 50k, that doesn't mean shit if he could 100k or not, Oda simply said fishmen are simply too weak, not that Rayleigh has equal CoC to Shanks. Both would KO all, don't mean shit other than Fishman Island Luffy is weak and Fishmen are weak.
Rayleigh couldn't even KO Law's underling Shachi, while Shanks sends CoC from 1 km to hurt an Admiral.