Ahead of every major GPU launch, both NVIDIA and AMD give out a document to reviewers known as Reviewer's Guide, in which both provide guidelines (suggestions, not instructions), to reviewers to ensure new GPUs are given a fair testing. In such documents, the two often also give out their own performance expectations from the GPUs they're launching, in which they compare the new GPUs to either previous-generation GPUs from their own brand, or from the competitors'. Apparently such a performance comparison between the upcoming Radeon HD 7970 and NVIDIA's GeForce GTX 580, probably part of such a document, got leaked to the internet, which 3DCenter.org re-posted. The first picture below, is a blurry screenshot of a graph in which the two GPUs are compared along a variety of tests, at a resolution of 2560 x 1600. A Tweakers.net community member recreated that graph in Excel, using that data (second picture below).

A couple of things here are worth noting. Reviewer guide performance numbers are almost always exaggerated, so if reviewers get performance results lower than 'normal', they find it abnormal, and re-test. It's an established practice both GPUs vendors follow. Next, AMD Radeon GPUs are traditionally good at 2560 x 1600. For that matter, the performance gap between even the Radeon HD 6970 and GeForce GTX 580 narrows a bit at that resolution.