No matter how good your QB, if he has bad protection and/or poor receivers, he will not do well. Most QB's will be very successful throwing short passes. The better the QB, the better he will perform under pressure and the more accurate he will be on longer passes. His accuracy and strength attributes + pressure drive the standard deviation of the speed and flight path of the ball; if a poor QB gets favorable dice rolls he'll throw lots of accurate passes, but the lower the bars and the greater the pressure, the less likely the QB will be accurate. The randomization is calculated using a normal distribution, so there is a bell curve in the middle for the most accurate pass, it's just that the lower their ratings the wider that bell is.
