Actually, the reverse is usually true.
Lower tire pressures result in a softer ride due to the tire being more compliant to bumps and such. Higher pressures yield a stiffer tire and a harsher ride.
But since you are using low-profile series tires, you don't have much room for tire compliance since there is less travel available from tread to rim...that is why a higher tire pressure works for you. With a low tire pressure, you probably have the tire literally hitting the inside of the wheel hub on hard bumps!
Gas mileage improves somewhat due to less heat and friction with higher tire pressures. And doing the 5 or so psi in your case was a cheap ride comfort formula for you!
