Not saying that in this situation it was correct... But with 3 crew onboard, maybe the decision was taken to have someone at the over wing exits instead of at the divider as there would only be 2 crew available to do the demonstration in the cabin, as the 3rd would need to read the PA from the back. Given if the divider was at maybe row 3, that would leave 19 rows of passengers trying to see the crew member at row 3...