I think that you hit the nail on the head when you said it was a 'gentle start' in Reception.
Reception class is supposed to cater for children aged 4 and 5. They do not, as others have said, like to sit still for any length of time. In my opinion, it is the teacher who has unrealistic expectations of the children she is teaching.
FWIW, my middle boy is a February birthday, so spot on average age for reception, and the teacher in reception was always telling me he wouldn't sit still/ concentrate/stop fidgeting on the carpet. When I dug deeper, I found he always came last to the carpet area, sat at the back/on the edge, and was disengaged. I suggested that the teacher asked him to come to the carpet first, and told him to sit near to her. She said it was his responsibility, and expected me to instill this in him
!
His teacher this year (Y1) has taken a different tack. She recognises that he is easily distracted and lives in his imagination, so she has given him his own piece of carpet to sit on at carpet time (keeps him in one place), and regularly engages him through questions, getting him to help, etc.
By the way, his little brother has only just turned 5 last week and is just finishing Reception. He's always first on the carpet, legs crossed, finger on lips, and sits still and listens for hours on end. the little goody goody . It's horses for courses, and a good teacher will meet all needs in the classroom.