It is RNG based but it is weighted and not in your favor regardless of some video posted on the internet by someone no one knows and did not show any of the testing. Anyone who uses the system can see it is not true RNG.
It works similarly to this:
RNG is set to generate a random number from 0-1000. There are 4 possible affixes.
0-500 you get option 1
501-850 you get option 2
851-950 you get option 3
951-1000 you get option 4.
It still uses random principles but the way it is weighted you will get option 1 or option 2 a vast majority of the time and will often see option 1 hit every roll. It is a cheap and underhanded way to waste time in game acquiring resources.