From the U.S. statement to the Rules Negotiating Group last week:
I’d like to address the issue that has been the focus of so many delegations here today – that is the issue of mandatory offsets or “zeroing.” Dumping margins occur when export prices are less than normal value, and requiring an offset for non-dumped transactions would result in an artificially lower margin. (emphasis added)
Of course, zeroing opponents say that zeroing results in an artificially higher margin. But higher or lower than what? What's the baseline? That's what the argument is fundamentally about.
There's also a reference in the U.S. statement to the prospective versus retrospective issue mentioned earlier on this blog, as well as some comments on issues other than zeroing.