A couple of months ago, a member here came screaming about his "bottleneck" number from that site.
A newer Ryzen system, with a GPU of (don't remember).
"18% bottleneck!! OH NO!"
I input my parts, just to see:
i7-4790k, RX-580 8GB
Result - 18% bottleneck.
hmm....interesting.
So I changed it to the GPU they suggested, a Quadrosomething. What?
Result - 39% bottlneck.
Yeah, OK.
That was a couple of months ago.
I tried that site again today.
Exact same parts, the i7, RX-580, 32GB RAM, SSDs.
Result - 9% bottleneck.
No parts changed.
I think they just have a poor RNG, and display whatever it spits up.