re: How did Carson Palmer end up as a journeyman QB in the NFL?

There are so many things it could be too. Maybe he lets go of the ball a bit too soon before getting hit, maybe he averts his eyes every 20 plays when he never used to do so in the face of a rush. These little things tally up the handful of incompletions and interceptions/sacks that separate the elite from the also-ran.

Many QBs aren't that "Far off" from being top QBs, but that slight difference matters when compiled over a season.