The hand in the set-position.

Most people have the thumb and the index finger close to the line during the set-position - the hands are rotated to the side.
I have however always felt this to be awkward and regardless of how I distribute weight between the arms and the legs the thumbs wants to give in and I have to concentrate hard to keep balanced at all.
If I rotate the hands slightly inwards however, so that the thumb end up further behind the line and the other fingers in front of it , everything is good.

Can anybody reason why this is? Strenght, flexibility etc…

This is not unusual. Ben did this very slightly most of the time. (see if they have any close-ups of the hands at the start)