I've booked a reservation for the first weekend of feb, with MD-80s both directions. On the flight out, It shows rows 9-11 ABD and 12 B as "Preferred seats". I'm in 12D on that flight (I don't have any status

). However, On the flight back (same plane), I was able to select 11D, and the only "Preferred seats" are 9-10ABD. So, does the number of these seats change based on the popularity of the route?