When we bought a new pack for my wife in 2013, the (obviously experienced) gearhead steered her towards a Gregory pack and away from an Osprey, pointing out that her natural back curvature was not very dramatic. 

Is this really a thing? I have noticed that Deuter and Gregory seem to have flatter harnesses than Osprey does. But I'm about to buy a new pack and haven't really noticed any discussion of this in my research.