I think you are underestimating the number of manufacturers and there volume of productions, Sapphire can produce around 100k pcs a week (first hand info, they don't even negotiate with traders for less then 20k pcs) not to mention that in that total hashrate there are tone of rx550/560 cards from 2014/15 so the number of gpus could be higher, stretch that on 3 year of manufacturing you get like approximately   400k GPU's a month this is easily achieved with all the vendors combined