I could be wrong and someone please correct me if so.
But I believe what they are talking about is the quickness of progression and regression. For instance Player X has A potential and Player Y has C potential. Player X will progress faster than Player Y. Once both guys reach their regression, or downhill side of their career, Player Y will regress faster than Player X.
It has little to do with what their current Overall says. In your case Axford has ooutperformed his potential and still may improve given his age. But he wont improve as much as a guy with A potential will and once he does start declining his decline will be sharper than Papelbons will due to their respective potentials.