I am no football coach, but I definitely see that Purdy does not move his feet well or move inside the pocket like the best QBs.

With his head coach supposedly a quarterback whisperer, I don't understand how it can be, but look at some video and you will see it. On many drop backs his feet are stationary as if in cement ("aimed" at his primary receiver as called in the huddle) rather than "chopping" like Manning, Brees, Brady, Stafford, etc.

This leads him to be "out of position" to throw most efficiently at times, especially if his 1st option isn't there and he needs to redirect and go to his 2nd.
He is very good at executing the play Shanahan has called as long as the primary/first receiver is open.

When he has to come off that first option, he either chucks the ball out of bounds/incomplete on purpose, tries to create by scrambling or tries to get to his 2nd read usually without a lot of success.

I think this is why you see him miss open receivers by large margins now and then.

Amazingly, Purdy having to go to his 2nd read does not happen very often. Is Shanahan that good at scheming the first guy open or am I missing something??

When I watch Brees, Manning, Stafford, I see "chopping" of feet and going from first option to second or third in split seconds somewhat frequently ... and accurately.