Because assuming the game is being ported from a PC engine, AF maybe just comes along in a format the xbox can use without changes? And maybe there isn't time (or priority) to make the changes to bring it to PS4. [... Is there any pattern in these games? Are they all directX on PC? Are they all using similar middleware?
The point being, with xbox one being directX based, maybe it is more forgiving of PC engines being ported to it, whereas PS4 needs a little more sanity checking for some pipelines. And so where AF may be slightly more likely to fall through the cracks on PS4
It has nothing to do with being ported from DirectX at that point, it's only a matter of GPU architecture/power. The graphical API, given the same feature set (which is pretty close here between PS4/DirectXOne) will only affect the CPU side of performance. AF will only affect GPU performance.
But there is a clear distinction between the GNMX and GNM API, correct? The former is DirectX friendly (hence the 'X') but highly inefficient; while the second entices developers with the promise of greater performance, at the expense of more work, because they are forced to unpick and code things in a different way. This is where a PS4 port often differs from its Xbox One counterpart. And due to time constraints, or the experience of the team doing the port, some things fall through the cracks. It also helps to explain why PS4 exclusives have no problem in this area.