The i7-3960X has the Sandy Bridge architecture...
What we're seeing here is one of the very few games than can take advantage of more than four cores.
My mistake the 3960 is SB but Borderlands2 doesn't look at it runs better with more cores at all, at least not always. Look at the graph, the 4100 runs faster than the 8150 and the 3770k or 2700k is barely faster than the 2500k.