There can be only one answer to this.It's weird how several times, such as this case the One is getting better than it's FLOPS advantage over the Pro, (here 2:1 resolution)
Is it the higher bandwidth? The thing is, 1X doesn't have 2X the bandwidth of Pro either. In fact, it's about 1.5X, same as the Flops advantage.
It has more RAM, but again it's exactly 50% more, not 2X.
It's probably an outlier, probably some games over no performance advantage on X, some over 30% more, some 2X, if you averaged them all it would probably come out around 50% as expected. I guess that would be my best guess.
So, hell if I know *shrug*.
Secret Sauce