One of the key fundamentals of sharp quarterback play is precise footwook. Christian Ponder and Matt Casell on the surface may have similar stats and appear to play the same typical West Coast game-manager style, however, it is their footwork that separates the two and consequently paints a picture of inconsistent play.
Cassel is a technician when it comes to his 5 step drops. The former Pro Bowl QB has sharp quick strides and he is able to consistently make his reads and release the ball a bit quicker than Ponder. I also think Cassel gets the hand off to the running backs slightly sooner as well. Now, with that said, after 5 steps all bets are off. Ponder is much better at stepping up in the pocket, throwing on the run or making a play when things break down.