thats the special gift of the chemical despots - normal doesnt exist any more.
i suspect that the heavier halogens must be dealt with first in order to get down to displacing fluoride. in a test tube, there must be over 7 times the amount of Iodine over fluoride in order to get past the gradient. thats by weight. only twice as much is needed to start mobilizing bromine. plus, i doubt there are very many places in the body that resemble the environment in a test tube.