But they are not the same certification. Electricians, plumbers, orthopedics, gynecologists, etc..., all have different training and certification. There are one man companies up to mega corporations so size is not a leading factor. In some professions there are layers in broad brush strokes, bachelors in whatever for example, but I see no reason there could not be something similar for different styles. There are specialty fields in every skilled trade I can think of so why can't there be "specialist" or unique styles within a given system?