I practice at home before school.
At that time, the handwriting with the left hand was better than that with the right hand.
If I write with my left hand, I can become a calligrapher in ten days; Then it will take me at least 1000 days to write with my right hand before I can become a calligrapher.
I feel that practicing my right hand is much more cost-effective than practicing my left hand.
But later, one was that the teacher didn't allow me to write with my left hand, and the other was that I wrote from left to right, so writing with my left hand would make my hands get ink or pencil ash. So I was forced to write with my right hand.
This is enough to show that people are not born with their right hands more flexible than their left hands, but because of their acquired behavior (often replacing their left hands with their right hands), their right hands become more flexible than their left hands.
At least when I was born, my right hand was far less flexible than my left.
I don't know if my left hand was more flexible than my right hand when I was born, or if everyone was born with his left hand more flexible than his right hand.