A 3900X is a 24 thread cpu, and 30% of that is around 7 full threads - of course, the game doesn't distribute the tasks that way.
I don't know how accurate using 2 different monitoring apps is either... they could be interfering with one another.
Im just wondering about the thing in the forza benchmark saying "limited gpu-percentage" beeing at 29-40%.
Afterburner and HW said, that the Power of the Gpu were max. at 75-80%. Does that help?
Wait a minute... a benchmark?
...
Oh wow. I had to look up "limited gpu-percentage" for this game.
That is just like one of those bottleneck calculators, and those aren't reliable at all.
The game benchmark is telling you that you need a faster cpu if you want even greater performance, and to leverage the gpu usage.
You aren't 'gpu limited' at all in this game. You are limited in the sense that the current cpu can't feed enough frames to the gpu to keep the 'limited gpu-percentage" at 100%.