As Airbus 340 points out, the standard economy seat in the A320 is
1) Standard Economy Seats
-Wider seat for improved passenger comfort
-62in triple seats seats
The main reason why this is standard is obviously comfort. But one thing that is rarely pointed out is that if the narrower seats are used, the passenger next to the window doesn't have much room because he tucked up under the sidewall, due to the A320 fuselage curvature. The A320 sports a round fuselage, meaning the widest point is approx seat bottom height. If you install the narrower seats, the window seat is too far outboard, at shoulder height.
The 737 has a double bubble fuselage, meaning the widest point is at the shoulder. The difference between the A320 and 737 at shoulder height is 2.8 inches, or less then half an inch per passenger.
Boeing offers 17.2 inch seat bottoms, where Airbus offers 18 inch seat bottoms (with standard 19 inch aisle).
""providing addl passenger comfort via slightly wider seats and/or a wider aisle""
Can't have both, it's either one or the other