"Once again, we see maximum on-board memory usage is higher on the Radeon RX 480 than the GeForce GTX 1060 6GB. The former reaches ~7GB, while the latter approaches 6GB."
This sounds linke you are trying to make a negative point towards AMD, or at least fail to point out the elephant in the Room, the 1060 6GB is worse in those two metrics. (in my opinion, and I am not a RedFanboy)
The facts and conclusions behind both memory usage and CPU usage differences are:
Memory: Ofc the 1060 !!6GB!! Does use less VRAM, it is maxed out! This is a BAD thing. The card basically has to swap to system memory at this point.
CPU: Obviously CPU usage for the 1060 6GB will be higher because the card is swapping to system memory. The CPU has to carry that overhead back and forth. This is a symptom of the card being maxed out with it's VRAM.
Conclusion: the 1060 has not enough VRAM and puts more strain on your CPU, which might not be equipped to handle that load. In that case it will cause frame drops at best and if the swapping task has a lower priority than the rendering pipeline it might even cause loading freezes, while the CPU swaps the data back and forth again.