The shape is functional for coaxing filament out of the edges with less strain on the AMS motor (in my view) as the lift and angle pull the slightly bound filament away from the neighbouring strands and freeing it up faster. Also, when AMS pulls back/unloads, it won't pull back tight to the spool again, which again has a good effect beyond the stated compatibility with spools with gaps that could hitch on a differently-shaped end. Simple and good; thanks.