I think it's impossible to prove anything. Still, not only the PS4 is clearly ahead in term of hardware, but also the XOne has a limitation in term of memory speed (for the main RAM), and memory size (for the quick ESRAM). So it really makes sense that the XOne is sub 1080p, and that the PS4 can output at 1080p what the XOne output a 900p.
In fact, it should not perform the same way. The only reason it could would possibly be a very intensive CPU task. That's what Ubisoft try to use as a reason when they say: "It's not the number of polygons that affect the framerate. We could be running at 100fps if it was just graphics, but because of AI, we're still limited to 30 frames per second."
What they are telling is that they give a priority to the AI so high, they have a target in term of AI so much higher than any game in the past in this genre, that they were OK to lower by a large margin the resolution, effects and framerate not just on the PS4, but also on the XOne. I call that a total bullshit.
As for the real reason, there is no problem having a higher resolution on a similar and better hardware. But it's impossible to say if they did that for MS, or for MS clients.
