Demand has to do with number of REA applications. I don’t know Harvard vs Stanford application numbers there. But I do know this----every time there was an unhooked Harvard admit in China there would be a newspaper article featuring the kid because it is perceived as rarer and more difficult (or “more newsworthy” I guess). I don’t recall seeing articles featuring kids admitted to other US schools including Stanford. Maybe a few years from now as more and more Chinese coming to study in US high schools and colleges the perception of Harvard vs Stanford will converge with that of US where H and S are neck and neck.
Another reason for this lopsided REA numbers could be that Harvard feels so confident about its brand appeal in China that it believes all 12 Stanford REAs will still apply to Harvard and Harvard can lure them away in RD.