The trouble with youtube videos is that they were recorded, uploaded, compressed and then played back at a constant rate. So even if the numbers look good the experience they were seeing and what you are watching aren't always going to match up.
Some games require more resources than others. FPS shooters often employ hitbox registration on various body parts so there is a lot of calculation involved for damage figures and sending that information off to other players, in the case of multiplayer. Add on top of that the data the GPU needs to process every frame and some CPUs will be pushed to the limit.
Assassin's Creed III is another title entirely with its own game engine and it will have different requirements. Given the single player nature, I suspect less CPU is required to maintain adequate FPS. I don't know a lot about it, but pretty much exclusive to the AC series and a very few others. (Didn't realize Rainbow Six Siege also uses it, I'll make a note of that)
Some very old benchmarks showed an FX-4100, and a ludicrous GTX690, getting an average of 33 FPS at 1920x1080p. At 900p, and with the slightly faster FX-4320, I would expect those 30-60 FPS numbers, but not consistently.