Macro lenses are generally one group 'lower' than their aperture would suggest. I suspect it's due to the apparent aperture decrease at macro distances. This has been the case for a while, long before the 5DIII/1D X - for example, the 180mm f/3.5 with the 1.4x TC will only AF with the center point of pre-1D X 1-series bodies, despite a reported aperture of f/5.