It almost has to be the product of a series of random number generations. I tried charting it early on and found no other rhyme or reason to answer why in some cases the 35 WE guy has more growth in the offseason than the 70+ WE guy while at other times the latter gains 20+ points in the offseason while the former basically comes back the same player with 4 fewer stamina points. The higher the WE, the more likely it seems that players will gain points in attribute categories in the off-season and the less likely they are to lose points, but it certainly isn't an assurance. Where those gains and losses occur also doesn't seem tied to potential level, so I'm pretty sure it's just a proverbial "roll of the dice" and stamina just has its own unique chart when it comes to that roll.