There is a lot of tesselation with hairworks and in the water and terrain (though may be bugged as it has limited visual impact), and by NVIDIA's own claims the efficiency of Maxwell is around 3x that of Kepler in this type of processing.
Sometimes raw power (which the 780 has) is needed to match more efficient processes (which for this usage it is claimed) that the 9xx series cards have.
I've seen this mentioned so many times.
I've yet to see any clear distinction between tessellation performance of Maxwell compared to Kepler. I've seen benchmarks for Unigine Heaven on extreme tessellation for example and it didn't appear to demonstrate this.
Kepler is also supposedly faster at tessellation than the Radeon 200 cards.
Another noteworthy point is that a lot of these benchmarks are with Hairworks off and the water in the game doesn't seem to have a noticable impact on performance at all. It can also be tested at minimal levels of tessellation and the performance advantages of Maxwell will still be noticably farther ahead than Kepler in W3.
Some other benchmarkers also brought up other games like Project Cars and GTA5 in this same subject of Kepler vs Maxwell performance as of late.
It could be that the drivers a while ago weren't utilising the Maxwell cards as well as they could, but I doubt it.