Its very archaic as are a lot of things this side of the pond. Back in the day there were the medically trained people, the Doctors and then there were the non medically trained surgeons, who, were generally and literally butchers (and barbers) so just had the title Mr. For some reason it still stands today although thankfully today surgeons are a little more adept at their craft than they were a few hundred years ago. Sorry I resorted to a generalisation of the fact that they both are in fact doctors!!