I just now read through the BOS WIKI entry and it says, "with roots in the American military and government-sponsored scientific community from before the Great War", but I can't tell its been retconed or just appended... (to incorporate FO3).
I don't know about 'government sponsored scientific community' but if you recall (Since I know you're a Fallout oldie) The BoS was formed by Roger Maxson and the base military personnel at Mariposa as well as the base families. Now, on that my assumption is basically this, it's likely in their own eyes, what with the idea of collecting and restoring technology, they, in their own eyes, saw themselves as 'Knights of Humanity' in the sense of the Brotherhood of Steel being who they are. So they cast off the US Military ranks and adopted more 'knightly' titles.