I've puzzled over that one for fifty years - but the ones without flanges are probably a lot cheaper to manufacture - much simpler forging. And even taking into account the cost of the flange, cost is still probably a lot cheaper. And flogging out of axle flanges was not a problem until oil lubricated bearings were phased out in the late 90s. For example, at nearly 600,000km I have replaced one flange on my 110.
John

