I disagree with the fact that the AMD specs are higher because the Dunia engine is based on Cry 1. They've known since at least 2011 that AMD was the GPU pick for PS4 and XB1, and that's a lot of potential customers and they know that. I'm sure the engine will run much better on AMD hardware. Yes I know the game is a PC install base, but there's no doubt Ubisoft is aiming at console gamers first; just look at this year's E3 showings from them.
Sure, disagree all you want. Programmers can and often will, when they don't care about the 'other hardware', specifically target a particular hardware set. If they specifically target Nvidia and optimize their code for running on Nvidia hardware the requirements are going to be much lower and utilize the resources much more efficiently than they would AMD or Intel GPUs. This comparison works for not only rendering graphics but CUDA/OpenCL as well. The hardware architecture structure is entirely different between Nvidia's kit and AMD's kit. Nvidia is generally less thread dependent and more about pumping through pipelines faster. AMD is much more multithreaded oriented so highly threaded rendering or computation loads are more efficient with theirs. It's the same with the CPUs. Intel targets single and low thread count programs. Their cores are designed to work in tandem while AMD's cores are independant and can take more independent threads, programs, processes.
Your post is self contradictory. First you say the specifications are higher because they are optimized heavily for Nvidia through the Cry engine, then turn around and say it will work 'better' on AMD hardware in the consoles. You can't have both. Either you optimize heavily for Nvidia's architecture or you optimize for AMD. You have to redesign and rewrite to change and that's far more difficult than non-programmers seem to believe. This is why most development houses either design for a single system, or if they design for multiple platforms, they only optimize as far as they practically can without overly damaging the results on either.
As for benchmarks like 3DMark and the like, Nvidia and AMD both have been caught optimizing for certain benchmarks so the results are skewed in their marketing favor and don't match the practical real world results. Take such benchmarks with much skepticism if at all.