[philjohn]

And it appears that for titles using Vulkan or DX12 AMD sees a bigger uplift than NVidia cards switching to those APIs for reasons unknown.

[TheGuyWhoCodes]

Reason are unknown but there are speculation. https://www.youtube.com/watch?v=nIoZB-cnjc0

This is a very good explanation/speculation which deals with the NV driver optimization for DX11 where they break up the draw calls between threads because the scheduler is software based where AMD is hardware based and can't do the same. In DX12 this isn't needed so AMD scheduler being hardware based can be better utilized.

[bryanlarsen]

NVidia has a killer driver team, which gives them a huge advantage. This advantage is lessened in the "lower-level" DX12 & Vulkan API's.

[floatboth]

And on the other hand, Graphics Core Next was initially designed for low level APIs — they actually started the whole "low level API on desktop" trend with Mantle.