I don't know this for a fact, but I suspect it's because you can actually get guys to go a point or two above their potential number during the season, but they always drop back down in the offseason.
As a comparison (if I remember correctly), before potential guys all "maxed out" at 97. You could get them up higher (99 or 100) during the season, but they would always drop back to 97 during the offseason.
Like I said, I don't have any inside info, but that's what I suspect.