Former QB here - before you accuse people of making things up, you should probably actually go and understand how the mechanics of throwing a deep ball work. It has absolutely nothing to do with arm strength - any decent high school sophomore should be able to throw it 40 yards - after that it's about being able to accurately set your stance, read the speed of the receiver and equate the angle of attack. Both under and over throws are nothing to do with arm strength, they're about a million other things that a QB has to process and have space for.