I have 2 Zons YSA and DYHA, so I can compare.
It's like having 2 different creatures in the house.
My YSA knows she is a bird while DYHA thinks she is a human.
It's a big puzzle for me too.
I can only say, my YSA is beautiful, smart, VERY sweet and VERY quiet parrot.
The only thing comes to mind is YSA's are not as good at talking. If someone is ready to get an Amazon parrot, they probably would rather go for the one that is going to talk.....I guess.
Marley is very quiet compare to Simba. She hardly ever raises her voice, usually only if I spend too much time with Simba or take Simba downstairs and leave Marley in the bird room. Sometimes when Simba gets really loud talking and singing, Marley is trying too, but her attempts sound more like screams........
Having YSA I can say she is a great pet bird, no doubt.
But they are rare.....I know that, because the Avian (and one of the best) Vet I took her to was not even sure what the heck she is until I told him.