Thank you, that makes sense.
Moreover, there are auto-trading services that track the prices among the exchanges and if there is a big difference they transfer funds and use the difference to gain some profit, so that all is like interconnected vessels in physics :) So querying all the possible exchanges would more or less be too much.
I found the steem price feed python script on github

The values in red on the steemworld witness page are those that fall outside some preset deviation, I suppose.