It's a glitch that is most apparent in Stamina, but others claim that they've seen it elsewhere (I'm not convinced). The glitch is that a recruit coming in at "Low-Low", may see their stamina decline over the summer, but the rate of improvement will not be reset.
The net effect is that Stamina will improve with practice, but only at an effective rate identical to that if they had only 1 point left until their cap (which is outrageously slow). I have even had a player turn from red back to black (so 7+ points of improvement remaining), but the rate of improvement only allowed them to gain 1 of those points back in their Senior season!
Evidently, this is a very difficult programming fix. I have suggested that the easy fix is similar to the band-aid put on initial ratings (i.e. that players with "blue" ratings would start no lower than 10); that incoming recruits always have at least 4 or 5 points of Stamina improvement remaining. I am also of the opinion that the programmers made offseason stamina ratings function differently than other categories. If that is a separate bit of code, then perhaps they could edit that separate section to rectify this issue, but they know about it.