Arm stregnth is subjective to a degree. If Rodgers and Luck stood on the goaline and both had to throw the ball as far as they could, Luck would probably throw further. But if u look @ them in the pocket needing to throw a ball 20yrds between a LB and safety in the tightest coverage possible, nobody throws a prettier ball w/ more velocity than Aaron Rodgers. Thats how I judge armstrenth. Velocity and accuracy on intermediate throws. Not who throws it the furthest.
Also Rodgers and Flacco display their arm strength in some of the worst weather imaginable. Not from the comfortable settings of a controlled environment.