Now that Y PAX can pre-book the seats at reservation time, AF matters in advance the perfect weight balance of the aircraft, notably on medium haul aircrafts which are more sensitive compared to bigger long haul planes.
On expected full plane this is not a problem but on half booked flights AF could reassign seats if all PAX are seating at the front.

For instance LH seems to be less pro-active on this weight balance mgmt . It happens sometimes that at boarding you get a new seat assigned at the back (even if you are SEN ! It happened to me) .

Of course various other factors can explain not bookable seats (extra seat, PETC, VIP++, COVID measures, Bassinet seats, etc)