that my friend is called arbitrage, basically meaning using the difference in price between 2 or more exchanges, for that one has to have funds both in (in this example) STEEM and BTC,
but I'd say most of the people go to the exchange with the highest volume to buy, cos they assume it is the safest one, since there's most of the money being traded, but then that's just my perspective, would be cool to hear some other thoughts bout that!