We have zero examples of quarterbacks in a Snyder system making a considerable jump in passing ability from their first to second years.
Literally never happened. Especially the guy with the funky motion that won the Unitas award.
This.
Here is a chart of all 2nd year Snyder QBs (and 3rd for Ell) and there is improvement in nearly every category. Note, this doesn't include the bowl game for Ertz, but his numbers are still very solid. Also, this accounts for the average improvement of all of these QBs in their 2nd years (+ Ell's 3rd) and its probably more likely Ertz surpasses the average if he stays healthy IMO.
I've always felt that it's mostly (not entirely) a byproduct of the coaching staff allowing the QB's to do more in the offense. The diversity of play calls in year two for all of these guys is much greater, and they're allowed more freedom to spread the ball down the field.
Of course, there's always the fact that we drill the crap out of them over the winter in the film room, they get added reps in the Spring and Summer as the clear #1, etc. But there is something systemic in what we do that accounts for this jump, and I remember how much they opened the book up for Ell, Klein, Waters, etc. in year two. I feel that's a pretty significant variable.