I think some of it may have to do with SAB, which used to have primarily Russian and French teachers who taught classical ballet. Now the teachers are primarily Americans, former NYCB dancers who teach the Balanchine style. Habits are learned very early in ballet, and if you're taught classical port de bras from the beginning, it won't ever completely go away. However, if a dancer only knows Balanchine port de bras, his/her arms will look quite different.