from the witness price feeds, and they get them from a mix of differenct exchanges where steem is listed.
that is one of the reasons to vote for a witness like @blockbrothers (witness team with @exyle) or @swisswitness (where I am part of)
you are voting for @blockbrothers already (which is a good choice) but you are voting for quite a few dead witnesses as well
Thanks!
I really need to update my witness list one of the next days...