I have run into this only twice on the flights I have taken out of STL. It was a 737 and it had the typical 3-2 seating layout with 2 exit rows over the wing.

On the seat backs of both exit rows there were tray tables and the arms rests were removable. I was always under the impression that trays were in the arm rests so that in an emergency the trays would not fall down and get in the way.