Heh, the secret is prolly stopping down of the lens. Most photographers know that stopping down a lens to f/8 or f/11 will produce the sharpest image. Canon L lens and nikon AFS-ED series are known to produce gd images even wide open, but still stopping them down 1 or 2 stops produce the sharpest image.
For camera lens 200mm or wider, it's easy to produce then at f/2.8 or faster. Let's say i were to get a 85mm f/1.8 lens, i can easily bring it down to f/4 and have super sharp images across the field.
When designing lens, optician have many factors to consider, and most of the time they have to compromise to produce the best lens for the situation.
In the case of the 400/4 and 600/4 lens, they have to consider the use for birding and sports event. Stuff like speed(aperture), focusing speed, image stablisation are far more important than chromatic aberration and sharpness.
The colour difference between a bird and the surrounding around it is often not so big, thus chromatic aberration is often unable to be seen. In the case of shooting stars, the stars and it's background is simply a torture for the lens.
One should also note, telescopes are meant to operate at over 50x the aperture, camera lens are just meant to image on film, and such fast lens are often matched with fast film, where the resolution isn't means to be so high. All along users would never discern any difference if the lens is any sharper.