Sounds to me that the overall "feel" of the ZL1 was better. Same reason why American cars have been losing to European cars in magazine tests for years, even when the American cars were faster...
The comfy ride of the ZL1 is exactly what is giving the win to it. It's ridiculous a muscle car shoot out puts comfort as the highest measure, while performance takes a backseat. The magazine tests are what they are and what they've always been.
And if I wanted a car for a comfortable ride, I would be looking at SUV's or luxury cars, not muscle cars.
Also, this is the first mag test where I've seen reported brake fade on the GT500.