I may be wrong here, but in my experience with franchise in the Show it seems like minor league players get potential buffs and nerfs more frequently. Its either the fact that they are in the minors or their ratings aren't high so theres more room for fluctuation, that I'm not sure of. It seems like they also go up and down based on streaks rather than prolonged stats like MLB players do. I've had guys in the MLB get boosts in potential but definitely not as often.
An example was Wily Peralta last year in my Brewers franchise. I believe his potential went up probably to around 85 after his 1st 20 starts because he was lights out. However minor league guys seem to go up or down based on the stats of a month or so.
In my opinion this is how it should be, a guy can fluctuate in the minors, but once he's gotten to the show he kind of stays true to his potential. It's pretty realistic because most guys that get to the majors are who they were when they got called up for most of their career, unless they completely fall apart or just blow up and go off.
Not sure if I'm the only one that's noticed this but its been true in every year I've played the show in my experience.