NewSong,
As far as I'm aware the ZR1 used by Autocar was a reader's car on loan and the GT2 (I think) was supplied by Porsche. I would imagine that in the Motortrend test the ZR1 will probably have been supplied by GM. So what I am basically suggesting is that one might be a little stronger in performance than the other.
Another thing I have noticed in different reviews conducted throughout the world is that depending where the review is and the cars involved the usual thing is that the car that's manufactured in the country of the review generally performs at it's best.
Why I have no idea.
BTW, my comments previously were not referring to the ZR1 vs GT2 conducted by Autocar but more a general opinion of Autocar's abilities which I rate as very good.