Soldato
Either ms helped them (new DX dev kit?) Or nvidia have replaced certain items in DX with their own implementation. It was never going to show the kinds of results mantle shows as mantle was built from the ground up with low overhead in mind, 337 is merely a patch for an existing api in DX. Regardless its showing some very healthy gains, long may it continue.
I'd say it's down to them removing support for all their old GPUs.
Now the drivers don't have to consider hardware 10 years old they can iron out a lot of the old slow tech.