jc - no, its not exponential.
i mostly agree with what iguana says there, but i dont really agree with that frame of mind that 18 minutes is 3 times the return of 10. with 10 minutes, you are not declining, and are also getting 4 minutes of growth. with 18 minutes, you are not declining, and are also getting 12 minutes of growth. but thats not really 3x, just one component is 3x, the other is the same. its like saying 6 minutes = 0 growth so 7 minutes is infinitely better.
but i agree with what he was getting at about 0 minutes. if you have red categories on a young player, its usually worth putting 0 minutes in to those categories (especially if they arent one of the 3-4 cores for that player), and losing 0 or maybe 1, or possibly even 2 points, while putting 20 in a critical blue rating. you might lose 1 point on average or something but in that blue core, you might go from +8 with 14m to +12 with 20m, and that is well worth it.
however, if you are talking about splitting minutes between 2 categories - say you have 30 minutes for bh/pass and both are blue. well, if we assume its 5m to hold it even, you definitely dont want to go 5/25. diminishing returns kick in significantly after 20m in an individual practice area, and i think they actually start to kick in soon, around 15m or so. so you would probably go 15/15m to safely optimize, thats a net 20m of growth, but you aren't hitting big diminishing returns like you are if you do 5/25. so in general the idea you are proposing in your post, its not right.
there are a number of reasons other freshman with similar work ethic could grow faster.
first, they could get more playing time, thats a big factor.
second, they could have higher potential in the categories they are practicing. growth rate in a category is a product of their work ethic, playing time, and their starting and maximum ratings (and of course, the amount of minutes out put in). so if they are improving a 50 starting rating with an 80 cap, they will gain more points than you do improving a 50 starting rating with a 65 cap, or a 10 starting rating with a 40 cap. that is why a lot of coaches, myself included, put as much time as they can into the cores with really high growth (the high/highs), because you get the most back. eventually it will all even out - but you are better off to have say +10 reb for the next 2 years than +5 pass, even though as a senior you will have practiced both and gotten your +10 reb and +5 pass (just for instance). so take the important ratings with a lot of room to grow, and put more minutes into them, and you can maximize growth.
third, they could have fewer minutes assigned to study hall and team practice than you, which results in more minutes going to individual practice, and hence, more growth. its valuable to have team practice minutes, but if you are lazy with your study hall minutes, maybe that is something you could look at. also, you might put more into FT - which would not show in the overall increase in your freshman - but its still valuable. so just looking at individual rating growth isn't really the complete picture- there is team practice, ft%, and the impact on your grades to consider as well.
finally, they could be getting players with lows, and putting 0 into all those lows, thus freeing up more minutes for the high growth areas.
some combination of those possibilities accounts for why other teams have more growth than you, although i cant say exactly which apply to you.