The difference is the location of the seats. The way it was explained to me, as you get close to the front and rear of the aircraft, the fuselage becomes a bit more narrow. The width of the aisle is an FAA minimum so the only place to "borrow" the lost room is from the width of the seats. The exit row is at the widest part of the cabin so there is room for the tray without cutting to much into the size of the seat or extending the row into the aisle. Up front, the difference is minimal but, I assume the company decided that the width of the seat would not be within their "acceptable" range. The narrowing of the cabin is more pronounced in the last row (23)...It can get pretty tight back there...and it never fails that on a full flight it is always 3 broad shouldered guys shoehorned in there with their arms and legs going numb. In the end, with regard to the first row...my guess is that inability to raise the armrest to accomodate disabled Customers was the decding factor in not adding armrest tray tables.